protoparse: oneof with only groups results in error

[jhump]

In #262, I added support for groups in oneofs. However, there was a latent bug in this functionality: if a oneof contains only groups (and no regular fields), then it results in an error during parsing: oneof must contain at least one field

Minimal reproducer:

message Foo { oneof bar { group Baz = 1 { optional int abc = 1; } } }

[jhump]

Fixed in #316