Create new blocks as valid

[aduth]

Regression introduced in #1929

This pull request seeks to resolve a bug where creating a new block will flag it as corrupt, presenting the user with a warning.

The issue is that we check ! block.isValid as considering a block to be corrupt, and this property was not assigned to newly created blocks.

We may want to refactor this behavior to explicitly check either false === block.isValid or block.isInvalid instead, but this change addresses the immediate breakage.

Testing instructions:

1. Navigate to Gutenberg > New Post
2. Click Write
3. Note you are not presented with corrupt warning

[nylen]

WFM 👍

A new block should be valid, and isValid can be thought of as a required attribute like uid and name. I don't see a problem with this fix.

[aduth]

(Publicly shames self for neglecting to squash & merge)

[aduth]

I think I needed to have regenerated the .json block fixtures, as the ordering of isValid changed.