Add input validation to prevent empty user messages in translation example

[mshsheikh]

Problem:
The script allows empty or whitespace-only input from the user, which results in unnecessary agent executions and invalid translations.

Changes:

• Added input validation to check if the user input is empty or whitespace-only.
• If invalid, raises ValueError with a clear message: "Input message cannot be empty."
• Prevents downstream agent execution when input is invalid.

msg = input("Hi! Enter a message, and we'll translate it to Spanish.\n\n")
if not msg.strip():
    raise ValueError("Input message cannot be empty.")

Benefits:

• Ensures only valid, non-empty input triggers agent workflows.
• Improves user experience by failing early with a descriptive error.
• Avoids unnecessary API calls or agent runs for invalid inputs.