TestFilter: Don't NPE if no methods are specified.

Avoid a NullPointerException if method == null
(i.e., the -m command line option isn't used).

Fixes commmit 3a7cf30e2301e7af41334c647dcc3a1190e4236d.

Change-Id: Ib439a892867be78ad67e313629871fecf33c90f7
diff --git a/tools/tradefed-host/src/com/android/cts/tradefed/testtype/TestFilter.java b/tools/tradefed-host/src/com/android/cts/tradefed/testtype/TestFilter.java
index edff85c..448f0675 100644
--- a/tools/tradefed-host/src/com/android/cts/tradefed/testtype/TestFilter.java
+++ b/tools/tradefed-host/src/com/android/cts/tradefed/testtype/TestFilter.java
@@ -88,7 +88,9 @@
      */
     public void setTestInclusion(String className, String method) {
         mIncludedClass = className;
-        mIncludedMethod = Pattern.compile(method);
+        if (method != null) {
+            mIncludedMethod = Pattern.compile(method);
+        }
     }
 
     /**