pipelines: Pipeline.Provider is optional, avoid segfault.
pipelines: Pipeline.Provider is optional, avoid segfault.

This was segfaulting in the existing test suite:

--- FAIL: TestPipelinesService_Update (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x6e0614]

buildkite/go-buildkite,wolfeidau/go-buildkite