[parquet-cli] dictionary command throw NPE when specified column isn't dictionary encoding

'dictionary' command of parquet-cli throw NPE when specified column isn't dictionary encoding.
```Java

$ java -cp 'target/classes:target/dependency/*' org.apache.parquet.cli.Main dictionary /work/parquet-mr/data/test.parquet -c binary_field
Unknown error
java.lang.NullPointerException
at org.apache.parquet.cli.commands.ShowDictionaryCommand.run(ShowDictionaryCommand.java:78)
at org.apache.parquet.cli.Main.run(Main.java:147)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:70)
at org.apache.parquet.cli.Main.main(Main.java:177)
```

The schema of 'test.parquet' is following:
```Java

$ java -cp 'target/classes:target/dependency/*' org.apache.parquet.cli.Main meta /work/parquet-mr/data/test.parquet

File path: /work/parquet-mr/data/test.parquet
Created by: parquet-mr version 1.12.0-SNAPSHOT (build 1e62e2e2ca903d4109480bc87ceec1dc954b6c92)
Properties:
writer.model.name: example
Schema:
message test {
required int32 int32_field;
required int64 int64_field;
required float float_field;
required double double_field;
required binary binary_field;
required int64 timestamp_field (TIMESTAMP(MILLIS,true));
}

Row group 0: count: 395 15.87 B records start: 4 total: 6.120 kB
--------------------------------------------------------------------------------
type encodings count avg size nulls min / max
int32_field INT32 _ D 395 0.20 B 0 "32" / "426"
int64_field INT64 _ D 395 0.20 B 0 "64" / "458"
float_field FLOAT _ _ 395 4.13 B 0 "1.0" / "395.0"
double_field DOUBLE _ _ 395 8.13 B 0 "2.0" / "396.0"
binary_field BINARY _ D 395 2.98 B 0 "0x6162636465666768696A6B6..." / "0x6162636465666768696A6B6..."
timestamp_field INT64 _ D 395 0.23 B 0 "2018-11-04T12:41:15.123+0000" / "2018-11-04T12:47:49.123+0000"

Row group 1: count: 395 15.92 B records start: 6271 total: 6.142 kB
--------------------------------------------------------------------------------
type encodings count avg size nulls min / max
int32_field INT32 _ D 395 0.20 B 0 "427" / "821"
int64_field INT64 _ D 395 0.20 B 0 "459" / "853"
float_field FLOAT _ _ 395 4.13 B 0 "396.0" / "790.0"
double_field DOUBLE _ _ 395 8.13 B 0 "397.0" / "791.0"
binary_field BINARY _ D 395 3.03 B 0 "0x6162636465666768696A6B6..." / "0x6162636465666768696A6B6..."
timestamp_field INT64 _ D 395 0.23 B 0 "2018-11-04T12:47:50.123+0000" / "2018-11-04T12:54:24.123+0000"

Row group 2: count: 234 16.53 B records start: 12560 total: 3.777 kB
--------------------------------------------------------------------------------
type encodings count avg size nulls min / max
int32_field INT32 _ D 234 0.17 B 0 "822" / "1055"
int64_field INT64 _ D 234 0.31 B 0 "854" / "1087"
float_field FLOAT _ _ 234 4.11 B 0 "791.0" / "1024.0"
double_field DOUBLE _ _ 234 8.21 B 0 "792.0" / "1025.0"
binary_field BINARY _ D 234 3.38 B 0 "0x6162636465666768696A6B6..." / "0x6162636465666768696A6B6..."
timestamp_field INT64 _ D 234 0.35 B 0 "2018-11-04T12:54:25.123+0000" / "2018-11-04T12:58:18.123+0000"
```

**Reporter**: [Masayuki Takahashi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=masayuki038) / @masayuki038
#### Original Issue Attachments:
- [test.parquet](https://issues.apache.org/jira/secure/attachment/12959433/test.parquet)

_{**Note**: *This issue was originally created as [PARQUET-1535](https://issues.apache.org/jira/browse/PARQUET-1535). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Tái hiện lỗi bằng lệnh dictionary của parquet-cli với test.parquet đính kèm, chọn float_field, trường không được mã hóa bằng dictionary. Kiểm tra ShowDictionaryCommand.java tại dòng 78 và điểm vào của lệnh để xác định giá trị null. Công việc được hoàn tất khi một cột không được mã hóa bằng dictionary không còn gây ra NPE và lệnh báo cáo điều kiện này một cách phù hợp.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.