Use packed encoding if there are any values, not just more than 1

[kellycampbell]

I found this via a problem with the JS protoc generated code which doesn't properly decode unpacked enums when the proto definition says the field is packed.

Here's what the protobuf documentation at https://developers.google.com/protocol-buffers/docs/encoding#packed says:

A packed repeated field containing zero elements does not appear in the encoded message. Otherwise, all of the elements of the field are packed into a single key-value pair with wire type 2 (length-delimited).

Here's an example of the JS error stack when this happened:

Error [AssertionError]: Assertion failed
    at new goog.asserts.AssertionError (node_modules/google-protobuf/google-protobuf.js:79:876)
    at Object.goog.asserts.doAssertFailure_ (node_modules/google-protobuf/google-protobuf.js:80:257)
    at Object.goog.asserts.assert [as assert] (node_modules/google-protobuf/google-protobuf.js:81:83)
    at jspb.BinaryReader.readPackedField_ (node_modules/google-protobuf/google-protobuf.js:261:71)
    at jspb.BinaryReader.readPackedEnum (node_modules/google-protobuf/google-protobuf.js:266:287)

[jhump]

@kellycampbell, this patch is fine.

However, you may also want to file a bug against the JS project, because the spec (just below the part you pasted) also states this:

Protocol buffer parsers must be able to parse repeated fields that were compiled as packed as if they were not packed, and vice versa.

[kellycampbell]

However, you may also want to file a bug against the JS project, because the spec (just below the part you pasted) also states this:

Protocol buffer parsers must be able to parse repeated fields that were compiled as packed as if they were not packed, and vice versa.

Thanks. I should have mentioned the JS project has an issue already reported on this here: protocolbuffers/protobuf#1701