fix(mutation): support conditional fetching by allowing falsy keys

[DiogenesFerminS]

Description

This PR aligns the behavior of useSWRMutation with useSWR regarding conditional fetching.

Previously, providing a null key (or a function returning a falsy value) to useSWRMutation caused an internal console.error during serialization and threw a "missing key" error when calling trigger(). This behavior contradicted the SWR documentation on conditional fetching, which states that falsy keys should simply prevent the request.

Changes

• Logic Fix: Added a guard clause in the trigger function to return early if the resolved key is falsy. This ensures consistency across the library and prevents unhandled console errors.
• Test Update: Updated the legacy test case should error when triggering an empty key to reflect the correct documented behavior (it now expects a silent return instead of an error throw).
• Bug Fix: Prevented the internal serialize utility from being called with invalid keys, eliminating the reported console noise.

Linked Issue

Fixes #4217

Testing

• Verified with a new regression test for null keys.
• Ran the full mutation test suite: pnpm test mutation. All tests passed.

Cc: @SL-YutoTakagi

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