bitbake: data_smart: Fix inactive overide accidental variable value corruption

Setting something like:

  BAR:append:unusedoverride

should cause BAR to be None, not "" which was what the datastore was
returning. This caused problems when mixing variables like:

  RDEPENDS:${PN}:inactiveoverride
  RDEPENDS:${BPN}

since key expansion would report key overlap when there was none. This
is a bug in the datastore. Fix it and add a test too.

[YOCTO #14088]

(Bitbake rev: 699e36c270d863258502d315ed00a1b940bfbf96)

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

bitbake/lib/bb/tests/data.py

@@ -405,6 +405,11 @@ class TestOverrides(unittest.TestCase):
         bb.data.expandKeys(self.d)
         self.assertEqual(self.d.getVar("VERSION"), "2")
 
+    def test_append_and_unused_override(self):
+        # Had a bug where an unused override append could return "" instead of None
+        self.d.setVar("BAR:append:unusedoverride", "testvalue2")
+        self.assertEqual(self.d.getVar("BAR"), None)
+
 class TestKeyExpansion(unittest.TestCase):
     def setUp(self):
         self.d = bb.data.init()