[mlir-opt] Fix dialect preload after fb1bb6a

Also pipe empty string to the commandline test to make sure it does
not hang on some configurations.

mlir/lib/Tools/mlir-opt/MlirOptMain.cpp

@@ -336,6 +336,7 @@ LogicalResult mlir::MlirOptMain(int argc, char **argv, llvm::StringRef toolName,
   // Parse pass names in main to ensure static initialization completed.
   cl::ParseCommandLineOptions(argc, argv, helpHeader);
   MlirOptMainConfig config = MlirOptMainConfig::createFromCLOptions();
+  config.preloadDialectsInContext(preloadDialectsInContext);
 
   // Set up the input file.
   std::string errorMessage;

mlir/test/mlir-opt/commandline.mlir

@@ -1,4 +1,4 @@
-// RUN: mlir-opt --show-dialects | FileCheck %s
+// RUN: echo "" | mlir-opt --show-dialects | FileCheck %s
 // CHECK: Available Dialects:
 // CHECK-NEXT: acc
 // CHECK-NEXT: affine