For UserSettingsController::UpdateDictionaryVariable(), clear the dictionary

if passed in a NULL new_value and the operation intended is eVarSetOperationAssign.
This fixed a bug where in TestSettings.py:

        # Set the run-args and the env-vars.
        self.runCmd('settings set target.process.run-args A B C')
        self.runCmd('settings set target.process.env-vars ["MY_ENV_VAR"]=YES')
        # And add hooks to restore the settings during tearDown().
        self.addTearDownHook(
            lambda: self.runCmd("settings set -r target.process.run-args"))
        self.addTearDownHook(
            lambda: self.runCmd("settings set -r target.process.env-vars"))

"settings set -r target.process.env-vars" was not restoring the original env-vars
setting.

llvm-svn: 116895