[Android Crash] skip region monitoring if map object is null

[mattshen]

This happens if google play services version is lower than required. It is to fix issues#618

There are also other places checking if map == null before calling startMonitoringRegion or stopMonitoringRegion usually followed with another call, e.g.

public void animateToRegion(LatLngBounds bounds, int duration) {
    if (map != null) {
      startMonitoringRegion();
      map.animateCamera(CameraUpdateFactory.newLatLngBounds(bounds, 0), duration, null);
    }
  }

However, both ways of check map's nullability are lack of elegance.

[dramaking]

Other region related method like "fitToCoordinates" need to do a null check as well. Otherwise the following error will come out:
"CameraUpdateFactory is not initialized"