Fix spurious CHECKJNI failure in hfpclient.

sendATCmdNative is allowed to have a null args parameter
but the jni code calls getUTF8Chars without checking
resulting in a checkjni failure on eng builds. Add a null
check around so that it doesn't fail this way.

Change-Id: I1432c6b528da1c53627694cc1c7a91c038b7cc24