Improve static array initializer checks

[ntrel]

~Disallow~ Deprecate null literal runtime SA initializer (except as an element initializer when element type accepts null). Rationale in bug below. Disallow init as runtime SA initializer when static array length > 0. Allow init as initializer for global static array regardless of length.

Fix Issue 23381 - init as initializer of 0-sized static array

[RazvanN7]

This requires a changelog entry.

[RazvanN7]

@ntrel Could you please add a changelog entry?

[RazvanN7]

Allow init as initializer for global static array regardless of length.

Why? I don't understand why this should be legal:

int[1] a3 = (int[]).init; // ok

(int[]).init is null. If int[1] a3 = null is disabled why allow this?

[RazvanN7]

ping @ntrel