Blocks: Save embed with empty markup when URL not selected

[aduth]

Fixes #2612

This pull request seeks to resolve an issue where a figure with text "undefined" is saved into post content for an embed block, if a URL is not yet assigned to the block.

Open questions:

In cases such as these where a block has a placeholder state, is it sensible to save the block at all, or would it make more sense to omit the block from the saved content? There may be technical challenges in implementing this behavior.

Testing instructions:

1. Navigate to Gutenberg > New Post
2. Insert an embed-type block (Vimeo, etc)
3. Preview the post
4. Note that the previewed post appears empty
5. Return to the editor
6. Refresh the page
7. Note that the embed block still exists in the post, in its placeholder state

[codecov]

Codecov Report

Merging #2616 into master will increase coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2616      +/-   ##
==========================================
+ Coverage   31.49%   31.49%   +<.01%     
==========================================
  Files         177      177              
  Lines        5398     5410      +12     
  Branches      943      948       +5     
==========================================
+ Hits         1700     1704       +4     
- Misses       3128     3132       +4     
- Partials      570      574       +4

Impacted Files	Coverage Δ
blocks/library/embed/index.js	45.55% <50%> (+0.1%)	⬆️
blocks/library/video/index.js	24.13% <0%> (+3.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c3b740d...14abbe7. Read the comment docs.

[youknowriad]

would it make more sense to omit the block from the saved content? There may be technical challenges in implementing this behavior

I think the answer to this is that we should save it (like now), There were some suggested use-cases where an author creates the placeholders and another one fill them.