External Media: Show for empty galleries

[obenland]

This PR fixes a regression introduced in d2c4f68, where External Media would not be available for empty galleries.

Fixes #16069 (comment)

Changes proposed in this Pull Request:

• Always treat value like an array and check for the amount of items it contains.

Does this pull request change what data or activity we track or use?

No.

Testing instructions:

• After applying this PR and building extensions, open the Editor.
• Insert any gallery-type block (gallery, tiled-galley, slideshow) and make sure External Media is available when the block is empty, and unavailable when it contains at least one image
• Insert any other image-type block (image, media/text, featured image, cover) and make sure External Media is always available, even when it contains an image.

Proposed changelog entry for your changes:

• None needed.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello obenland! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D45504-code before merging this PR. Thank you!
This revision will be updated with each commit to this PR

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

E2E results is available here (for debugging purposes): https://jetpack-e2e-dashboard.herokuapp.com/pr-16288

Scheduled Jetpack release: July 7, 2020.
Scheduled code freeze: June 30, 2020

Generated by 🚫 dangerJS against 2212091

[retrofox]

LGTM

empty gallery

not empty gallery

It doesn't open the external media modal.

[jeherve]

This tests well for me. 🚢

[obenland]

r209630-wpcom