Added a conditional check for unreachable vertices

[simran0963]

Problem

• Problem with the condition checking for unreachable vertices

Solution

• Added a conditional check for vertices that are not reachable

Changes proposed in this Pull Request :

• 1.
• added for vertex, distance in shortest_distances.items(): if distance == float('infinity'): print(f'Shortest distance from {start_vertex} to {vertex} is unreachable') else: print(f'Shortest distance from {start_vertex} to {vertex} is {distance}')