Fix allocationless anonymous splat keyword argument check

Previously, if an argument splat and keywords are provided by
the caller, it did not check whether the method/proc accepted
keywords before avoiding the allocation. This is incorrect,
because if the method/proc does not accept keywords, the
keywords passed by the caller are added as a positional
argument, so there must be an allocation to avoid mutating
the positional splat argument.

Add a check that if the caller passes keywords, the
method/proc must accept keywords in order to optimize.
If the caller passes a keyword splat, either the
method/proc must accept keywords, or the keyword splat must
be empty in order to optimize.

If keywords are explicitly disallowed via `**nil`, the
optimization should be skipped, because the array is mutated
before the ArgumentError exception is raised.

In addition to a test for the correct behavior, add an
allocation test for a method that accepts an anonymous splat
without keywords.

Fixes [Bug #21757]

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