[ZEPPELIN-6507] Return empty list from FileInterpreter.completion()

[dani1552]

What is this PR for?

FileInterpreter.completion() returns null rather than an empty list. Per Effective Java Item 54, methods that return a collection should never return null, since it forces every caller to add a special-case null check.

In practice this base implementation is not reached today: the only concrete subclass, HDFSFileInterpreter, overrides completion() with a real implementation. However, if a future FileInterpreter subclass omits that override, the null would propagate through the completion call chain, which does not perform null checks.

This PR changes the base implementation to return Collections.emptyList() and adds a unit test that pins the contract.

What type of PR is it?

Improvement

Todos

• - Return Collections.emptyList() from FileInterpreter.completion()
• - Add testCompletionReturnsEmptyListInsteadOfNull to FileInterpreterTest

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-6507

How should this be tested?

• mvn test -pl file -Dtest=FileInterpreterTest
• TestFileInterpreter, the test double already present in FileInterpreterTest, does not override completion(), so the new test exercises the base implementation directly.
• All 5 tests in FileInterpreterTest pass locally.

Screenshots (if appropriate)

N/A

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No