Data binding: make sure field_name is a string

[pkevan]

Otherwise a fatal error occurs when nothing is configured.

Fixes #556.

[github-actions]

Test this PR in WordPress Playground.

[chriszarate]

@pkevan This doesn't quite track for me. Where was the fatal being thrown?

> php -a
Interactive shell

php > $foo = [];
php > $bar = $foo['foo'] ?? '';
php > var_dump( empty( $bar ) );
bool(true)

[chriszarate]

Ah, I suppose it's when it's passed to BlockBinding::get_block_fallback_content(). We could adjust the type hint there, but this works too.

[pkevan]

Ah, I suppose it's when it's passed to BlockBinding::get_block_fallback_content(). We could adjust the type hint there, but this works too.

Apologies - missed that part in the description!