Cover Text Block: Prevent class=false when serializing the block

[youknowriad]

Props @aduth for catching that. The Cover Text block was serializing class="false" with default Attributes which was causing the "externally modified content" message.

[codecov]

Codecov Report

Merging #2363 into master will increase coverage by 1.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2363      +/-   ##
==========================================
+ Coverage   24.95%   26.11%   +1.16%     
==========================================
  Files         153      156       +3     
  Lines        4780     5157     +377     
  Branches      803      906     +103     
==========================================
+ Hits         1193     1347     +154     
- Misses       3032     3179     +147     
- Partials      555      631      +76

Impacted Files	Coverage Δ
blocks/library/table/index.js	36.36% <ø> (ø)	⬆️
blocks/library/paragraph/index.js	47.05% <ø> (ø)	⬆️
blocks/library/image/index.js	16.43% <ø> (+4.19%)	⬆️
blocks/library/embed/index.js	52.02% <100%> (+6.57%)	⬆️
blocks/library/cover-text/index.js	35% <100%> (ø)	⬆️
components/button/index.js	90% <0%> (-0.91%)	⬇️
editor/modes/visual-editor/block.js	0% <0%> (ø)	⬆️
...ditor/modes/visual-editor/invalid-block-warning.js	0% <0%> (ø)	⬆️
editor/index.js	0% <0%> (ø)	⬆️
components/icon-button/index.js	100% <0%> (ø)	⬆️
... and 9 more

---

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 a0714e2...2973409. Read the comment docs.

[aduth]

This looks good, and we ought to do an audit of class name assignments like this, since I was able to confirm in minimal case that ReactDOMServer with stringify with false verbatim:

> const React = require( 'React' );
undefined
> const e = React.createElement;
undefined
> const ReactDOMServer = require( 'react-dom/server' );
undefined
> ReactDOMServer.renderToStaticMarkup( e( 'div', { className: false }, 'hello world' ) );
'<div class="false">hello world</div>'

There's another just one line below the one changed here.

[aduth]

It's primarily an issue in cases where the falsey value is false or 0. undefined or null will avoid assigning the class name.

Alternatively, we can encourage classnames, which handles this well:

> const classnames = require( 'classnames' );
undefined
> classnames( 'foo', false, 'bar', 0, 'baz' );
'foo bar baz'