XMLParserConfiguration forceList initial empty tag should be consistent with default processing

[stleary]

See discussion in #1039: #1039 (comment)

When a tag is in forceList, and the first element is empty, the first element is dropped, but the default non-forceList behavior is to insert an initial empty string. ForceList and default processing in this case should have the same result.

Although this breaks existing behavior, it is a corner case of XML parsing, and I believe it should be fixed.
Note that after the fix is made, in the case where there is only one instance of an empty tag, default processing results in an empty string value, but forceList processing will create an array with a single empty string as content. This is acceptable.

Fixing this will break several existing tests in XMLConfigurationTest.java:
testEmptyTagForceList()
testEmptyForceList()