Network: Check parameter for NULL in isMmsRequest method

It's the NullPointerException which causes the third part
application can't work normally. It happens when the third
part application calls execute in AndroidHttpClient.java.
The parameter may null when calling checkMmsSendPermission.

Add extra judgement to avoid NullPointerException for fixing
this issue.

CRs-Fixed: 584775

Change-Id: I1db5160952f1effeff7c35b10ae26656dfe476d8