UpdateOccurrence should check that the field mask is set

[alexashley]

In Grafeas, the mask is plucked off the request and passed to the storage layer without validation, so it can be nil. That causes this panic when the mask is updated:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xfa4252]
grafeas_1        |
goroutine 32145 [running]:
github.com/rode/grafeas-elasticsearch/go/v1beta1/storage.(*ElasticsearchStorage).UpdateOccurrence(0xc0005a3840, 0x13b6280, 0xc0007ab0e0, 0xc000536409, 0x4, 0xc00053641a, 0x24, 0xc000117000, 0x0, 0x4, ...)
	/workspace/go/v1beta1/storage/elasticsearch.go:397 +0x952
github.com/grafeas/grafeas/go/v1beta1/api.(*API).UpdateOccurrence(0xc0000c60a0, 0x13b6280, 0xc0007ab0e0, 0xc000432a00, 0xc0000c60a0, 0xc0007ab0e0, 0xc00028cba0)
	/go/pkg/mod/github.com/grafeas/[email protected]/go/v1beta1/api/occurrence.go:220 +0x2a4
github.com/grafeas/grafeas/proto/v1beta1/grafeas_go_proto._GrafeasV1Beta1_UpdateOccurrence_Handler(0x11de320, 0xc0000c60a0, 0x13b6280, 0xc0007ab0e0, 0xc000155080, 0x0, 0x13b6280, 0xc0007ab0e0, 0xc00047c480, 0x21e)
	/go/pkg/mod/github.com/grafeas/[email protected]/proto/v1beta1/grafeas_go_proto/grafeas.pb.go:2267 +0x217
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00036d880, 0x13c67b8, 0xc000244d80, 0xc000111100, 0xc0002f6e10, 0x1c18378, 0x0, 0x0, 0x0)
	/go/pkg/mod/google.golang.org/[email protected]/server.go:1210 +0x52b
google.golang.org/grpc.(*Server).handleStream(0xc00036d880, 0x13c67b8, 0xc000244d80, 0xc000111100, 0x0)
	/go/pkg/mod/google.golang.org/[email protected]/server.go:1533 +0xd0c
google.golang.org/grpc.(*Server).serveStreams.func1.2(0xc0000b8d10, 0xc00036d880, 0x13c67b8, 0xc000244d80, 0xc000111100)
	/go/pkg/mod/google.golang.org/[email protected]/server.go:871 +0xab
created by google.golang.org/grpc.(*Server).serveStreams.func1
	/go/pkg/mod/google.golang.org/[email protected]/server.go:869 +0x1fd

Either this function could return with an error or initialize an empty mask if it's not set. Either way, malformed requests shouldn't crash the entire server, see #99