fix oob read on empty array initializer in convertInitializerList

[saddamr3e]

ASan, on a shader with a nested empty initializer list:

SEGV on unknown address 0x000000000000
ParseHelper.cpp:9987 in glslang::TParseContext::convertInitializerList

The array branch reads getSequence()[0] to derive inner array dimensions but never checks the initializer list is non-empty. A nested {} (an empty EOpNull aggregate, legal under GL_EXT_null_initializer) that lands on an array element type reaches it with a zero-length sequence, so the index runs past the end. The struct/matrix/vector branches already validate the count first, and the HLSL convertInitializerList guards the same read with size() > 0. Reject the empty list with an error to match.

Repro: int a[1][3] = { {} }; with GL_EXT_null_initializer. Test added under gtests AST (glsl.nullInitializer.error.vert).

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[arcady-lunarg]

@saddamr3e I think you'll need to re-sign the CLA because they changed to a different CLA.