Don't pass in empty arg to dex2oat.

We would occassionally pass in an empty arg "" instead of a NULL.
This caused problems since dex2oat isn't made to handle empty args.

(cherry picked from commit f53dbfeda3)

Change-Id: Ia2d60b895391bf83b5d250c20ddb8a4917d347a6

cmds/installd/commands.c

@@ -623,7 +623,7 @@ static void run_dex2oat(int zip_fd, int oat_fd, const char* input_file_name,
     execl(DEX2OAT_BIN, DEX2OAT_BIN,
           zip_fd_arg, zip_location_arg,
           oat_fd_arg, oat_location_arg,
-          dex2oat_flags,
+          strlen(dex2oat_flags) > 0 ? dex2oat_flags : NULL,
           (char*) NULL);
     ALOGE("execl(%s) failed: %s\n", DEX2OAT_BIN, strerror(errno));
 }