ActiveJob: removing redundant if statement on Arguments serialization of String

[jarenas9539]

Motivation / Background

After changes from #50090 and #50122, a redundant if statement remains on Arguments#serialize on String related classes serialization. A comment related to this was added here.

Detail

When the begin/rescue statement was added to the else clause the if clause began to be scoped on the rescue clause as StringSerializer is not defined and will throw SerializationError (same current behaviour as other String subclasses without defined serializers). As if clause and rescue clause share same result, the if clause started to be redundant.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[skipkayhil]

As if clause and rescue clause share same result, the if clause started to be redundant.

While this may be true, I don't think we should make this change. Using rescue for control flow is much worse for performance than a simple if, and since I imagine String is a very common argument to serialize this could have a large impact on applications.