Allow Dialects to be initialized via nullptr.

[fedelebron]

Summary

This allows Dialect to follow the MLIR style of nullable objects, and in fact is expected by Dialect::operator bool() const which already tests whether def == nullptr. This just wasn't a reachable situation, because the constructor was dereferencing the pointer unconditionally.

[rriddle]

Please add justification in the description for why. Seems like this could be handled wherever Dialect is being constructed.

[rriddle]

Ah, I see that it already has an operator bool. Please still add the justification though. It's just as important to know why a change is happening as it is to know what is changing.

[fedelebron]

Done. Could you submit the commit please? Thanks!

mlir/lib/TableGen/Dialect.cpp

 //===----------------------------------------------------------------------===//
 
 #include "mlir/TableGen/Dialect.h"
 #include "llvm/TableGen/Record.h"
 
 using namespace mlir;
 using namespace mlir::tblgen;
 Dialect::Dialect(const llvm::Record *def) : def(def) {
+  if (def == nullptr)
+    return;
   for (StringRef dialect : def->getValueAsListOfStrings("dependentDialects"))
     dependentDialects.push_back(dialect);
 }
 
 StringRef Dialect::getName() const { return def->getValueAsString("name"); }
 
 StringRef Dialect::getCppNamespace() const {
   return def->getValueAsString("cppNamespace");