Don't pass empty arguments to ninja

Passing an empty argument in argv to ninja results in an error:
ninja: error: empty path

Don't add jarg to the argument list if it is empty.

Bug: 34392351
Test: m -j
Change-Id: I39c8998c79b09835650628e1413ae7625cdbc555
diff --git a/tools/makeparallel/makeparallel.cpp b/tools/makeparallel/makeparallel.cpp
index b906681..0e1e45c 100644
--- a/tools/makeparallel/makeparallel.cpp
+++ b/tools/makeparallel/makeparallel.cpp
@@ -338,13 +338,17 @@
       // ninja is parallel by default, pass -j1 to disable parallelism if make wasn't parallel
       args.push_back(strdup("-j1"));
     } else {
-      args.push_back(strdup(jarg.c_str()));
+      if (jarg != "") {
+        args.push_back(strdup(jarg.c_str()));
+      }
     }
     if (keep_going) {
       args.push_back(strdup("-k0"));
     }
   } else {
-    args.push_back(strdup(jarg.c_str()));
+    if (jarg != "") {
+      args.push_back(strdup(jarg.c_str()));
+    }
   }
 
   args.insert(args.end(), &argv[2], &argv[argc]);