fix empty parameter fatal in protoc-gen-grpc-gateway

[danielhochman]

Fixes #1751

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Split() on an empty parameter set for protoc-gen-grpc-gateway would cause a fatal since the for loop woulds still execute with an empty string.

Fixed with an early return and added some small tests.

F1014 11:23:06.564537  195394 main.go:115] Cannot set flag 
goroutine 1 [running]:
github.com/golang/glog.stacks(0xc000628000, 0xc0001ae680, 0x3c, 0x40)
        /home/danielhochman/go/pkg/mod/github.com/golang/glog@v0.0.0-20160126235308-23def4e6c14b/glog.go:769 +0xb8
github.com/golang/glog.(*loggingT).output(0xb8c4c0, 0xc000000003, 0xc0001fbf80, 0xb5bcf1, 0x7, 0x73, 0x0)
        /home/danielhochman/go/pkg/mod/github.com/golang/glog@v0.0.0-20160126235308-23def4e6c14b/glog.go:720 +0x372
github.com/golang/glog.(*loggingT).printf(0xb8c4c0, 0x3, 0x843bf2, 0x12, 0xc000295bc8, 0x1, 0x1)
        /home/danielhochman/go/pkg/mod/github.com/golang/glog@v0.0.0-20160126235308-23def4e6c14b/glog.go:655 +0x14b
github.com/golang/glog.Fatalf(...)
        /home/danielhochman/go/pkg/mod/github.com/golang/glog@v0.0.0-20160126235308-23def4e6c14b/glog.go:1148
main.parseFlags(0xc0001b4750, 0x0, 0x0)
        /home/danielhochman/go/src/github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway/main.go:115 +0x444
main.main.func1(0xc00039d5e0, 0x0, 0xc0001fa000)
        /home/danielhochman/go/src/github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway/main.go:68 +0x79
google.golang.org/protobuf/compiler/protogen.run(0xc0001977a0, 0x0, 0xc000295f30, 0xb8ce20, 0x7f326cb7ee98)
        /home/danielhochman/go/pkg/mod/google.golang.org/protobuf@v1.25.0/compiler/protogen/protogen.go:76 +0x142
google.golang.org/protobuf/compiler/protogen.Options.Run(0xc0001977a0, 0x0, 0xc000251f30)
        /home/danielhochman/go/pkg/mod/google.golang.org/protobuf@v1.25.0/compiler/protogen/protogen.go:54 +0x5a
main.main()
        /home/danielhochman/go/src/github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway/main.go:66 +0x2d1
--grpc-gateway_out: protoc-gen-grpc-gateway: Plugin failed with status code 255.

Other comments

[google-cla]

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[johanbrandhorst]

Thanks for this! Could you please run the bazel generator from the contribution guidelines to fix the test failure?

[google-cla]

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[danielhochman]

@johanbrandhorst done, sorry about that. Also I should have CLA signed within next 24 hours hopefully. Waiting on some internal review for our corp CLA.

[danielhochman]

@googlebot I signed it!

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

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[johanbrandhorst]

I think this will change soon with the work @adambabik is doing, but thanks for this quick hold-over fix.