fix: null pointer exception from PgResultSetMetaData when there's no column metadata

[IvyDev0]

As described in #1613 and a previous PR #663, the driver throws null pointer exception when building the results for queries without column metadata, e.g. SELECT <x> FROM <table name> <x>, where <x> is not a column name but an alias of the table. It should just return the whole table. (There might be other queries that can also reproduce this issue.)

However the above PR only fix one place where field.getMetadata() is null. There are four more places might cause the NPE.
In this minor fix, we return "" as the columnName, schemaName, tableName if the metadata is null, and ResultSetMetaData.columnNullable for nullable.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests?
2. Does mvn checkstyle:check pass ?
3. Have you added your new test classes to an existing test suite in alphabetical order?

Changes to Existing Features:

• Does this break existing behaviour? If so please explain.
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

[AppVeyorBot]

✅ Build pgjdbc 1.0.529 completed (commit d0dc351 by @)

[davecramer]

Thanks!

[davecramer]

Now I'm wondering why I closed it ?