fix!: encoding of empty Buffers

[vicb]

closes #1500
fixes #885

The fix is from @jawid-h in #1500. Thanks to him.
I added tests.

Note the change in behavior:

-                test.same(obj.bytesVal, [], "should set bytesVal");
+                test.same(obj.bytesVal, protobuf.util.newBuffer(0), "should set bytesVal");

The default value for an empty Buffer is now a Buffer of length 0 vs an empty array before.

I have not included the generated files in this CL (dist/* and index.d.ts).
Please let me know if they should be there and I'll add them.

[alexander-fenster]

We talked a little bit and decided it's a breaking change and should go to v7.0 which hopefully will be released soon enough.

[vicb]

Thanks for the update @alexander-fenster.

I think it's definitely safer to release this in a major version as there might be users relying on the current behavior.