Fix panic when reading file with no row groups (#408)

file.go

@@ -174,6 +174,10 @@ func OpenFile(r io.ReaderAt, size int64, options ...FileOption) (*File, error) {
 // this case the page index is not cached within the file, programs are expected
 // to make use of independently from the parquet package.
 func (f *File) ReadPageIndex() ([]format.ColumnIndex, []format.OffsetIndex, error) {
+	if len(f.metadata.RowGroups) == 0 {
+		return nil, nil, nil
+	}
+
 	columnIndexOffset := f.metadata.RowGroups[0].Columns[0].ColumnIndexOffset
 	offsetIndexOffset := f.metadata.RowGroups[0].Columns[0].OffsetIndexOffset
 	columnIndexLength := int64(0)