[C-API / JS-API] Binaryen crash if recent "memoryName" is null for memory ops

[MaxGraey]

After #4811 and #4940

If not specify recently introduced memoryName (just use nullptr) parameter for BinaryenLoad, BinaryenStore, BinaryenMemoryFill etc., this lead to crash with assertion inside emscripten-optimizer/istring.h (line 65):

RuntimeError: Aborted(Assertion failed: s, at: .../binaryen.js/binaryen/src/emscripten-optimizer/istring.h,65,IString).

https://github.com/WebAssembly/binaryen/blob/main/src/emscripten-optimizer/istring.h#L65