EmitC: Allow arrays of size zero (#123292)

This is allowed as a GCC extension, see
https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html.

mlir/docs/Dialects/emitc.md

@@ -16,6 +16,8 @@ The following convention is followed:
     floating types.
 *   If `__bf16` is used, the code requires a compiler that supports it, such as 
     GCC or Clang.
+*   If `emitc.array` with a dimension of size zero is used, then the code
+    requires [a GCC extension](https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html).
 *   Else the generated code is compatible with C99.
 
 These restrictions are neither inherent to the EmitC dialect itself nor to the