[mypy] Fix type annotations for graphs (TheAlgorithms#4622)

* Fix mypy error for frequent_pattern_graph_miner.py

* Fix mypy error for markov_chain.py

Author: imp2002

Diff for: graphs/frequent_pattern_graph_miner.py

@@ -227,6 +227,6 @@ def preprocess(edge_array):
     support = get_support(cluster)
     graph = construct_graph(cluster, nodes)
     find_freq_subgraph_given_support(60, cluster, graph)
-    paths = []
+    paths: list = []
     freq_subgraph_edge_list = freq_subgraphs_edge_list(paths)
     print_all()

Diff for: graphs/markov_chain.py

@@ -35,6 +35,7 @@ def transition(self, node: str) -> str:
             current_probability += self.connections[node][dest]
             if current_probability > random_value:
                 return dest
+        return ""
 
 
 def get_transitions(