Update dijkstra.cpp

[krishnaawasthi-26]

Description of Change

Added input validation for edge cases in Dijkstra’s algorithm implementation.

Specifically, handled the following conditions:

If vertices <= 0, output a message and terminate.

If edges < 0, output a message and terminate.

If edges == 0, output that all nodes are isolated and terminate.

These changes improve the robustness of the program by preventing invalid graph input scenarios and providing informative user feedback.

[krishnaawasthi-26]

please all this ---- Fixes #2958