[FLINK-27268][JDBC Connector] fix build sql query error in JdbcDynamicTableSource

[choucmei]

What is the purpose of the change

Fix When user query single contant,build query error in a jdbc connector table.

Brief change log

• add condition , when build query with none field, will query ''

Verifying this change

This change is already covered by existing tests, such as (org.apache.flink.connector.jdbc.statement.FieldNamedPreparedStatementImplTest#testSelectStatementWithNoneField).

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[flinkbot]

CI report:

• 7d9f981 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[MartijnVisser]

We've now moved the code from the JDBC connector to its own dedicated repository; please re-route this PR to https://github.com/apache/flink-connector-jdbc