map -e with no cert to PRESIGNED

Needed for backwards compatibility with scripts that use -e.
diff --git a/tools/releasetools/sign_target_files_apks b/tools/releasetools/sign_target_files_apks
index 4ec9347..91e03f4 100755
--- a/tools/releasetools/sign_target_files_apks
+++ b/tools/releasetools/sign_target_files_apks
@@ -91,6 +91,8 @@
 
   # apply all the -e options, overriding anything in the file
   for apk, cert in OPTIONS.extra_apks.iteritems():
+    if not cert:
+      cert = "PRESIGNED"
     certmap[apk] = OPTIONS.key_map.get(cert, cert)
 
   return certmap