Functions error if IOpipe is disabled and using context.iopipe

[pselle]

If using label or other context.iopipe methods, if a user disables the library, the function will error on a type error: TypeError: Cannot read property 'label' of undefined

Steps to reproduce the issue:

1. Instrument a function using context.iopipe.label or other methods
2. Disable the agent in config or env var
3. Function errors

Describe the results you received:
Function errors

Describe the results you expected:
Function should not error, or library should document how to approach this issue

Additional information you deem important (e.g. issue happens only occasionally):