[SPARK-10005] [SQL] Fixes schema merging for nested structs

In case of schema merging, we only handled first level fields when converting Parquet groups to `InternalRow`s. Nested struct fields are not properly handled.

For example, the schema of a Parquet file to be read can be:

```
message individual {
  required group f1 {
    optional binary f11 (utf8);
  }
}
```

while the global schema is:

```
message global {
  required group f1 {
    optional binary f11 (utf8);
    optional int32 f12;
  }
}
```

This PR fixes this issue by padding missing fields when creating actual converters.

Author: Cheng Lian <lian@databricks.com>

Closes #8228 from liancheng/spark-10005/nested-schema-merging.

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