If oem_fingerprint_props is empty, ignore.

There may be cases where there is an OEM partition
but it has no effect on the OTA itself.  In these
cases, ignore an empty value from the misc_info.txt

Change-Id: I5f467e873030765af12810a07ddd5f302ca8cc0b
diff --git a/tools/releasetools/ota_from_target_files b/tools/releasetools/ota_from_target_files
index 6f34450..e8dff5a 100755
--- a/tools/releasetools/ota_from_target_files
+++ b/tools/releasetools/ota_from_target_files
@@ -379,7 +379,7 @@
 
 def AppendAssertions(script, info_dict, oem_dict = None):
   oem_props = info_dict.get("oem_fingerprint_properties")
-  if oem_props is None:
+  if oem_props is None or len(oem_props) == 0:
     device = GetBuildProp("ro.product.device", info_dict)
     script.AssertDevice(device)
   else:
@@ -463,7 +463,7 @@
 
   oem_props = OPTIONS.info_dict.get("oem_fingerprint_properties")
   oem_dict = None
-  if oem_props is not None:
+  if oem_props is not None and len(oem_props) > 0:
     if OPTIONS.oem_source is None:
       raise common.ExternalError("OEM source required for this build")
     script.Mount("/oem")
@@ -746,7 +746,7 @@
 
   oem_props = OPTIONS.target_info_dict.get("oem_fingerprint_properties")
   oem_dict = None
-  if oem_props is not None:
+  if oem_props is not None and len(oem_props) > 0:
     if OPTIONS.oem_source is None:
       raise common.ExternalError("OEM source required for this build")
     script.Mount("/oem")
@@ -1036,7 +1036,7 @@
 
   oem_props = OPTIONS.info_dict.get("oem_fingerprint_properties")
   oem_dict = None
-  if oem_props is not None:
+  if oem_props is not None and len(oem_props) > 0:
     if OPTIONS.oem_source is None:
       raise common.ExternalError("OEM source required for this build")
     script.Mount("/oem")