[mlir] NFC - Fix OperationSupport.cpp::findNamedAttr

Summary:
When NamedAttrList::get is called against a StringRef and the
entry is not present, the Identifier::operator== crashes.
Interestingly there seems to be no use of the NamedAttrList::get(StringRef) in
the codebase so far. A subsequent commit will introduce such a use.

Differential Revision: https://reviews.llvm.org/D80089