Fix default unit issue for tag cloud block

[akasunil]

Fixed https://github.com/WordPress/gutenberg/issues/58885

What?

Tag cloud block default size is set in PT unit, but when PT unit is not available, it vanish on unit change.

Why?

Converting default value into first available unit.

also modified parseQuantityAndUnitFromRawValue() to return default unit value when match not found. Before it was returning undefined even when match isn't found.

Testing Instructions

• Add a Tag cloud block.
• In the settings panel, in the options "smallest size" and "largest size", change the unit.