bugfix(grpc) Handle null value in proto definition

[ghost]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

The proto definition produced by @grpc/proto-loader may have null value field, which will cause error.

syntax = "proto3";

message ProtoMessage {
    string param = 0x1;
}

The above message will generate following proto definition, which has null value field options.

{
  ProtoMessage: {
    format: 'Protocol Buffer 3 DescriptorProto',
    type: {
      field: [{
        name: 'param',
        extendee: '',
        number: 1,
        label: 'LABEL_OPTIONAL',
        type: 'TYPE_STRING',
        typeName: '',
        defaultValue: '',
        options: null, // <-- null value here
        oneofIndex: 0,
        jsonName: ''
      }],
      nestedType: [],
      enumType: [],
      extensionRange: [],
      extension: [],
      oneofDecl: [],
      reservedRange: [],
      reservedName: [],
      name: 'ProtoMessage',
      options: null
    }
  }
}

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 1407

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 93.826%

---

Totals
Change from base Build 1406:	0.003%
Covered Lines:	2872
Relevant Lines:	2995

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1407

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 93.826%

---

Totals
Change from base Build 1406:	0.003%
Covered Lines:	2872
Relevant Lines:	2995

---

💛 - Coveralls

[kamilmysliwiec]

LGTM. Thanks!

[lock]

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