[ADT] Deprecate MutableArrayRef(std::nullopt) (#146113)

ArrayRef(std::nullopt) just got deprecated.  This patch does the same
to MutableArrayRef(std::nullopt).  Since there are only a couple of
uses, this patch does migration and deprecation at the same time.

mlir/lib/IR/Diagnostics.cpp

@@ -727,7 +727,7 @@ SourceMgrDiagnosticVerifierHandlerImpl::computeExpectedDiags(
     raw_ostream &os, llvm::SourceMgr &mgr, const llvm::MemoryBuffer *buf) {
   // If the buffer is invalid, return an empty list.
   if (!buf)
-    return std::nullopt;
+    return {};
   auto &expectedDiags = expectedDiagsPerFile[buf->getBufferIdentifier()];
 
   // The number of the last line that did not correlate to a designator.