don't create RECORD when it would have no fields

[dmccartney]

given a proto like

message FooTable {
  option (gen_bq_schema.table_name) = "foo_table";
  string foo_field_a = 1;
  EmptyFooField foo_field_b = 2;
}

message EmptyFooField {
}

the plugin would produce an invalid schema:

...
{"name":"foo_field_b","type":"RECORD","mode":"NULLABLE"},
...

-> BigQuery error in mk operation: Field foo_field_b is type RECORD but has no schema

So this PR discards any RECORD that would have no fields.

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[dmccartney]

I signed it!

[googlebot]

CLAs look good, thanks!

[yugui]

LGTM. Thank you for your contribution.