[V3] Return null on IntSynth when null is passed in

[ejonasson]

The IntSynth in Livewire V3 adds the ability to convert an empty string that's passed into the synth to null. This is great on the frontend (since a form input will generally return '' instead of null, but there are still scenarios where null might be returned. For example, in a unit test (as demonstrated in the test on this PR), or when updated though Javascipt.

This PR just makes sure we pass though that null value in those cases, as opposed to passing it to the int cast, which converts it to 0.

1️⃣ Is this something that is wanted/needed? Did you create a discussion about it first?
Did not create a discussion, but an explanation is included above.

2️⃣ Did you create a branch for your fix/feature? (Master branch PR's will be closed)
Yes

3️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.
The only changes are to the IntSynth class, and adding a test for that Synth.

4️⃣ Does it include tests? (Required)
Yes

5️⃣ Please include a thorough description (including small code snippets if possible) of the improvement and reasons why it's useful.
Included above :)

[joshhanley]

@ejonasson thanks for the PR! I just pulled it down, and commented out your fix and the test still passes. Can you confirm the same happens for you?

[joshhanley]

@ejonasson actually don't worry, I managed to fix it 🙂

[ejonasson]

@joshhanley Oops, sorry about that. Thank you for fixing!

[calebporzio]

Thank you!