gcli: crash in generated CLIs with map fields (#538)
gcli: crash in generated CLIs with map fields (#538)

When a message contains a map<string,string> field, the generated CLI
allows map values to be set by accepting multiple options of the form:
	--parent.mapname "key=value"

However, when these options are provided the CLI crashes because the
generated CLI attempts to store them in a map that hasn't been created
yet. The map insertions crash because the map field is nil.

This commit adds code that creates maps if one or more of the options
illustrated above is provided. With these maps, the associated API calls
are made successfully.
googleapis/gapic-generator-go,googleapis/gapic-generator-go