gn2bp: Remove empty srcs cc_object from blueprint

* This was failing the pre-submits are cc_objects are compilable modules but they failed to compile due to empty srcs. However, this didn't affect the compilation of Cronet as the empty cc_objects are never in the path of compilation of Cronet because their attributes are propagated up the tree.

Test: m
Change-Id: I7cc87d53d4f460ef4958d5ecfb23ac28662fdbdc
diff --git a/tools/gn2bp/gen_android_bp b/tools/gn2bp/gen_android_bp
index 8a3c38b..c750c2a 100755
--- a/tools/gn2bp/gen_android_bp
+++ b/tools/gn2bp/gen_android_bp
@@ -531,7 +531,12 @@
 
   def to_string(self, output):
     for m in sorted(self.modules.values(), key=lambda m: m.name):
-      m.to_string(output)
+      if m.type != "cc_object" or m.has_input_files():
+        # Don't print cc_object with empty srcs. These attributes are already
+        # propagated up the tree. Printing them messes the presubmits because
+        # every module is compiled while those targets are not reachable in
+        # a normal compilation path.
+        m.to_string(output)
 
 
 def label_to_module_name(label):