Full Site Editing: Avoid throwing warnings if there are no terms for a template or template part

[Copons]

Description

Fixes #27172

A recent change added the wp_theme taxonomy to the wp-admin lists of templates and template parts.
I didn't consider that get_the_terms returns false instead of an empty array if it doesn't find anything.

After the many changes on the wp_template and wp_template_part custom post types, we currently assign the current theme slug to wp_theme on templates and parts creation.
Therefore, new templates would never trigger this warning.

While older templates (without wp_theme taxonomy) are barely supported now, it's still appropriate to make sure the data is valid before looping through it.

How has this been tested?

• Open both /wp-admin/edit.php?post_type=wp_template and /wp-admin/edit.php?post_type=wp_template_part.
• Make sure everything works as expected, and the items have the theme name in the Theme column.
• If you have older templates/parts, without wp_theme tag, make sure there are no warning in the PHP log.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: +254 B (0%)

Total Size: 1.2 MB

Filename	Size	Change
build/block-editor/index.js	133 kB	+254 B (0%)
build/block-library/index.js	148 kB	+2 B (0%)
build/blocks/index.js	48.1 kB	-2 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.8 kB	0 B
build/api-fetch/index.js	3.42 kB	0 B
build/autop/index.js	2.84 kB	0 B
build/blob/index.js	664 B	0 B
build/block-directory/index.js	8.71 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/style-rtl.css	11.3 kB	0 B
build/block-editor/style.css	11.3 kB	0 B
build/block-library/editor-rtl.css	8.96 kB	0 B
build/block-library/editor.css	8.96 kB	0 B
build/block-library/style-rtl.css	8.23 kB	0 B
build/block-library/style.css	8.23 kB	0 B
build/block-library/theme-rtl.css	792 B	0 B
build/block-library/theme.css	793 B	0 B
build/block-serialization-default-parser/index.js	1.87 kB	0 B
build/block-serialization-spec-parser/index.js	3.06 kB	0 B
build/components/index.js	172 kB	0 B
build/components/style-rtl.css	15.3 kB	0 B
build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.95 kB	0 B
build/core-data/index.js	14.8 kB	0 B
build/data-controls/index.js	827 B	0 B
build/data/index.js	8.8 kB	0 B
build/date/index.js	11.2 kB	0 B
build/deprecated/index.js	768 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.92 kB	0 B
build/edit-navigation/index.js	11.2 kB	0 B
build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
build/edit-post/index.js	306 kB	0 B
build/edit-post/style-rtl.css	6.45 kB	0 B
build/edit-post/style.css	6.44 kB	0 B
build/edit-site/index.js	23.6 kB	0 B
build/edit-site/style-rtl.css	3.86 kB	0 B
build/edit-site/style.css	3.86 kB	0 B
build/edit-widgets/index.js	26.4 kB	0 B
build/edit-widgets/style-rtl.css	3.13 kB	0 B
build/edit-widgets/style.css	3.13 kB	0 B
build/editor/editor-styles-rtl.css	476 B	0 B
build/editor/editor-styles.css	478 B	0 B
build/editor/index.js	43.3 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.85 kB	0 B
build/element/index.js	4.62 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	6.86 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.27 kB	0 B
build/html-entities/index.js	623 B	0 B
build/i18n/index.js	3.57 kB	0 B
build/is-shallow-equal/index.js	698 B	0 B
build/keyboard-shortcuts/index.js	2.54 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.1 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.32 kB	0 B
build/notices/index.js	1.81 kB	0 B
build/nux/index.js	3.42 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.43 kB	0 B
build/priority-queue/index.js	790 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/reusable-blocks/index.js	2.92 kB	0 B
build/rich-text/index.js	13.4 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.05 kB	0 B
build/viewport/index.js	1.86 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[Addison-Stavlo]

This is working for me! 👍

[gziolo]

I saw this error today for templates created before the taxonomy refactor was introduced 👍

[gziolo]

🚢