bitbake/data_smart: Don't export deleted/empty entries in the list of keys

If you d.delVar(), you expect the variable to be gone. Even empty
variables continue to exist in the datastore and are still user visible
unfortunately. The COW siutation means you can't just remove it
since it might unmask a variable from an inner copy.

This patch therefore stops empty variables from appearing in key lists
exposed to the external world making empty variables an internal
implementation detail only.

(Bitbake rev: 2b5548c591d4cfde9238d2cc0959c42cfc08f09c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/data_smart.py

@@ -406,6 +406,8 @@ class DataSmart(MutableMapping):
             for key in d:
                 if key == "_data":
                     continue
+                if not d[key]:
+                    continue
                 klist.add(key)
 
             if "_data" in d: