GetBootImageBuildProp returns None if file does not exist

If system/etc/ramdisk/build.prop does not exist in the ramdisk,
GetBootImageBuildProp should return None instead of a path
to a non-existant file.

Test: pass
Bug: 177171706
Change-Id: I34e447bc315d203e136e8c0d59d00961f81301e3
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 19b132b..26c4ae8 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -3672,14 +3672,13 @@
     RunAndCheckOutput(['toybox', 'cpio', '-F', abs_uncompressed_ramdisk, '-i'],
                cwd=extracted_ramdisk)
 
-    prop_file = None
     for search_path in RAMDISK_BUILD_PROP_REL_PATHS:
       prop_file = os.path.join(extracted_ramdisk, search_path)
       if os.path.isfile(prop_file):
-        break
+        return prop_file
       logger.warning('Unable to get boot image timestamp: no %s in ramdisk', search_path)
 
-    return prop_file
+    return None
 
   except ExternalError as e:
     logger.warning('Unable to get boot image build props: %s', e)