JSONPb marshaler panics if input is nil interface

[jhump]

The JSONBuiltin marshaler emits "null" when marshaling a nil interface, but JSONPb panics:

panic: reflect: call of reflect.Value.Interface on zero Value

goroutine 6 [running]:
testing.tRunner.func1(0xc4201982d0)
  /usr/local/go/src/testing/testing.go:711 +0x2d2
panic(0x1506500, 0xc4201bee20)
  /usr/local/go/src/runtime/panic.go:491 +0x283
reflect.valueInterface(0x0, 0x0, 0x0, 0x1, 0x0, 0xc4201cf340)
  /usr/local/go/src/reflect/value.go:936 +0x1bf
reflect.Value.Interface(0x0, 0x0, 0x0, 0x0, 0x0)
  /usr/local/go/src/reflect/value.go:931 +0x44
github.com/grpc-ecosystem/grpc-gateway/runtime.(*JSONPb).marshalNonProtoField(0xc420173f80, 0x0, 0x0, 0x0, 0x0, 0xc4201ccd00, 0xc4201cf368, 0x1)
  /Users/jh/go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/marshal_jsonpb.go:81 +0x59e
github.com/grpc-ecosystem/grpc-gateway/runtime.(*JSONPb).Marshal(0xc420173f80, 0x0, 0x0, 0x1, 0xc4201ccd01, 0x1, 0x0, 0x0)
  /Users/jh/go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/marshal_jsonpb.go:27 +0x16f
github.com/grpc-ecosystem/grpc-gateway/runtime_test.TestJSONPbMarshalFields(0xc4201982d0)
  /Users/jh/go/src/github.com/grpc-ecosystem/grpc-gateway/runtime/marshal_jsonpb_test.go:126 +0xe6
testing.tRunner(0xc4201982d0, 0x15da798)
  /usr/local/go/src/testing/testing.go:746 +0xd0
created by testing.(*T).Run
  /usr/local/go/src/testing/testing.go:789 +0x2de

[jhump]

@achew22, @yugui: we're in the process of converting some REST APIs to use protos, not all of which actually go through grpc-gateway generated handlers, but manual handlers that already return non-proto objects to serialize to JSON. So we're starting to use grpc-gateway's marshaler to aid in the transition, since it can marshal both protos and non-protos.

But we have some code that uses nil as a sentinel response, expecting it to be serialized as "null". That works with encoding/json, but caused grpc-gateway's JSONPb marshaler to panic. This fixes the panic.

[codecov-io]

Codecov Report

Merging #639 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #639      +/-   ##
==========================================
+ Coverage   58.88%   58.91%   +0.02%     
==========================================
  Files          30       30              
  Lines        2853     2855       +2     
==========================================
+ Hits         1680     1682       +2     
  Misses       1010     1010              
  Partials      163      163

Impacted Files	Coverage Δ
runtime/marshal_jsonpb.go	70% <100%> (+0.61%)	⬆️

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