[mlir] Set intial value for integer FieldParser function

GCC complains FieldParser returns the value that does not have an
obvious initial value. Set an initial return value for the FieldParser
function.

The change is a no-op functionality-wise. The return value should be
updated within the function.

This should fix https://github.com/llvm/llvm-project/issues/64312

Differential Revision: https://reviews.llvm.org/D156848