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.

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[AppVeyorBot]

✅ Build pgjdbc 1.0.529 completed (commit d0dc35101e by @)

[davecramer]

Thanks!

[davecramer]

Now I'm wondering why I closed it ?