Fix: only set initialData with demo initialHtml example if initialData not set

[mzorz]

Fixes a problem first observed by @jd-alexander in wordpress-mobile/WordPress-Android#12939 (comment), produced by the changes introduced in #2690

To test:
demo app:

1. Run npm run start:reset from the gutenberg-mobile folder
2. Run npm run core ios or npm run core android to open the demo app
3. Expect to see Jetpack blocks followed by core blocks

WPAndroid/WPiOS apps:

1. opening an existing gutenberg Post should not show the demo content but the actual Post's content

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.

[chipsnyder]

Thanks for taking care of this!
Once this merges we can close #2702

[mzorz]

Thanks for tying that issue @chipsnyder !

[guarani]

Thanks for fixing! I overlook this during #2690.

Checking now and will approve in a sec.

[chipsnyder]

This looks good! Tested on the demo apps and main apps for each platform.

[guarani]

I think this can be merged, I haven't had a chance to test but don't want to delay it.

[mzorz]

Thank you both @chipsnyder @guarani ! 🙇