block.json without a `supports` key throws notice

[walbo]

When WordPress converts typography keys declared under supports.* to supports.typography.* is never checks if supports is defined and throws an undefined index on blocks without a supports key.

Steps to reproduce:

• Create a block without adding supports to the block.json and register the block using register_block_type_from_metadata.

Trac ticket: https://core.trac.wordpress.org/ticket/53416

[SergeyBiryukov]

In 51167:

Editor: Check if supports metadata key is defined before migrating typography keys.

This avoids an "Undefined index" PHP notice on blocks without a supports key in block.json.

Follow-up to [51089], [51153], [51159].

Props walbo.
Fixes #53416.