Deprecation compliance: null as array offset

[KaseyJenkins]

• Change $a[null] to $a[''] to conform with PHP 8.5 deprecation.
• Test still passes today; this update future-proofs and aligns with the deprecation.
• No change in expected output; intent made explicit.

[TimWolla]

From what I see, this test is specifically designed to test the behavior with different data types, making this PR incorrect.

[iluuu1994]

The question rather is why the JIT doesn't emit this deprecation. That discrepancy should be fixed.

[iluuu1994]

/cc @Girgias. ext/opcache/tests/jit/fetch_dim_r_001.phpt should emit a deprecation, but doesn't.

[Girgias]

@alexandre-daubois wrote the implementation for this in #19511, I'm happy to investigate and attempt a fix for the issue if he doesn't want to do it.

[alexandre-daubois]

I'm having a look, I'll keep you updated. Thanks for the ping!

[alexandre-daubois]

Let's close this one as the fix has been merged. Thanks for the report @KaseyJenkins!