Filter.createSubstringFilter with "subInitial" or "subFinal" set to an empty string about ldapsdk

[dirmgr]

The LDAP SDK's API for constructing substring filters was created with the assumption that if a subInitial, subAny, or subFinal component wasn't needed, then it wouldn't be provided rather than given as an empty string. This is more in line with how filters are transmitted over the wire (they aren't sent as strings, but rather use a binary encoding using ASN.1 BER), and in that case it's illegal to provide substring components if the value is empty.

However, I realize that it's much more likely for developers to be more familiar with the string representations of substring filters, and it's understandable for someone to think that filters are provided to the LDAP server using their string representation, and in that case, it may make sense to assume that providing an empty string is equivalent to not providing one at all.

I went ahead made a couple of changes to the API. I have updated the documentation to indicate that the components should be null or non-empty strings, but also to treat empty strings as equivalent to null in this case. That change should be included in the next release (currently slated for sometime in December), but the workaround, and more correct behavior, would be to use null instead of empty strings.