fix: use from_networkx if graph does not exist in db

Author: aMahanna

nx_arangodb/convert.py

@@ -138,11 +138,11 @@ def _to_nxcg_graph(G: Any, as_directed: bool = False) -> nxcg.Graph:
             return G
 
         if isinstance(G, nxadb.Graph):
+            logger.debug("converting nx_arangodb graph to nx_cugraph graph")
+
             if not G.graph_exists_in_db:
-                m = "nx_arangodb.Graph does not exist in ArangoDB. Cannot pull graph."
-                raise ValueError(m)
+                return nxcg.convert.from_networkx(G)
 
-            logger.debug("converting nx_arangodb graph to nx_cugraph graph")
             return nxadb_to_nxcg(G, as_directed=as_directed)
 
         raise TypeError(f"Expected nx_arangodb.Graph or nxcg.Graph; got {type(G)}")