fix(m365 receiver): Add nil check to shutdown

[dpaasman00]

Proposed Change

The collector panicked with a nil pointer dereference if it was unable to create an authorization token when first started. This occurred because the attempt to create the token happens inside Start() before a cancel func is assigned to the recv. The collector would error out of Start() and then call Shutdown() and encounter the nil pointer dereference when trying to call cancel(). This PR adds a nil check on the cancel funcs before they are called, allowing the collector to fail correctly and not panic with a nil ptr dereference.

Checklist

• Changes are tested
• CI has passed