[mlir][python] Fix possible use of variable use before set

The _mlirRegisterEverything symbol may not be built by some customers. The
code here was intended to support this, but didn't properly initialize the
init_module variable.
This would break JAX with:

NameError: free variable 'init_module' referenced before assignment in enclosing scope

Author: joker-eph

mlir/python/mlir/_mlir_libs/__init__.py

@@ -83,6 +83,7 @@ def process_initializer_module(module_name):
 
     # If _mlirRegisterEverything is built, then include it as an initializer
     # module.
+    init_module = None
     if process_initializer_module("_mlirRegisterEverything"):
         init_module = importlib.import_module(f"._mlirRegisterEverything", __name__)