ResolvedContext.get_resolved_package() should return None for failure

Signed-off-by: Alexey Pelykh <alexey.pelykh@gmail.com>

src/rez/resolved_context.py

@@ -438,7 +438,7 @@ def get_resolved_package(self, name):
         """Returns a `Variant` object or None if the package is not in the
         resolve.
         """
-        pkgs = [x for x in self._resolved_packages if x.name == name]
+        pkgs = [x for x in (self._resolved_packages or []) if x.name == name]
         return pkgs[0] if pkgs else None
 
     def copy(self):