Fix save lock control (length check on object)

[glingener]

Description

Tried to use #10649 and found a little glitch using .length on an object, which broke the whole feature.

How has this been tested?

Using wp.data.dispatch( 'core/editor' ).lockPostSaving( 'mylock' );
to lock the publish/update button

And wp.data.dispatch( 'core/editor' ).unlockPostSaving( 'mylock' );
to unlock it.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[aduth]

Nice catch, thanks. I pushed updated tests in cfdc3b7, as the previous ones were obviously insufficient in capturing the original issue.