Bug 2210 NS instantiation fails in basic12

An exception is raised during NS instantation
when we try to access vnf_profile_id of
a NS charm (VCA) which is None

Change-Id: I7e3a26c6877f117b4ad5834d2b1a86767e7b7756
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>
diff --git a/osm_lcm/data_utils/vca.py b/osm_lcm/data_utils/vca.py
index 0f3ec17..ff52d22 100644
--- a/osm_lcm/data_utils/vca.py
+++ b/osm_lcm/data_utils/vca.py
@@ -224,3 +224,7 @@
     @property
     def config_sw_installed(self) -> bool:
         return self.get("config_sw_installed", False)
+
+    @property
+    def target_element(self) -> str:
+        return self.get("target_element", "")