Exit early from onClick handler when isButtonDisabled

[adamsilverstein]

Description

Fixes #11809
Alternate/replaces #11760

Return early from click action handling when button is disabled (isButtonDisabled is true)

Since #11543 the post publish/update button is no longer disabled when isButtonDisabled is true (https://github.com/WordPress/gutenberg/pull/11543/files#diff-29409e4e3f779a199509ffadad068564L76) - instead aria-diabled is set to true (https://github.com/WordPress/gutenberg/pull/11543/files#diff-29409e4e3f779a199509ffadad068564L76) which still allows the button itself to be clicked.

This PR prevents the button from being clicked by returning early from the onClick handler, see @youknowriad's comment here: https://github.com/WordPress/gutenberg/pull/11760/files#r236177807

How has this been tested?

Needs testing.

Types of changes

Add a check for isButtonDisabled in the onClick handler, exit early if true - preventing click action handling.

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.

[adamsilverstein]

thanks for the review... not sure why checks are failing here, @youknowriad they pass for me locally

[youknowriad]

The tests are broken across the repository (some weird docker failures), @gziolo is working on them, we should rebase once everything is on track again.

[adamsilverstein]

we should rebase once everything is on track again.

👍 I'll wait for that then merge once I see tests passing.

[gziolo]

You can rebase now. Travis is back to normal.