SpanUtil.toRowColumn(Key) may have bug when family is empty and qualifier is not

I noticed that `SpanUtil.toRowColumn(Key)` ignores the qualifier when the family is empty. Maybe this intended for the use within Span, not sure. However for other places that use this code, it may cause problems.

Start at SpanUtil.toRowColumn(Key), the only entry point named in the issue, and inspect how it handles an empty family with a non-empty qualifier. Compare its use within Span with other callers to determine the intended result; no file or test is named, so completion requires confirming the expected behavior and adding appropriate regression coverage.

Written by the indexing model from the issue text.