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GH-40249: [Java] Fix NPE in ArrowDatabaseMetadata (#40988)

### Rationale for this change

When retrieving database metadata using the JDBC driver, some data such as SQL keywords could be null. Before this change, an NPE would be thrown when trying to convert the list of SQL keywords into a String.

### What changes are included in this PR?

The following database metadata fields:

* SQL keywords
* Numeric functions
* String functions
* System functions
* Time/date functions

will convert to an empty string when they are null.

### Are these changes tested?

A unit test has been added to verify that the fields above are converted to the empty string when null, with no exceptions thrown.

### Are there any user-facing changes?

The fields above will now return an empty string rather than throw an NPE.
* GitHub Issue: #40249

Authored-by: Norman Jordan <norman.jordan@improving.com>
Signed-off-by: David Li <li.davidm96@gmail.com>