Regression: Image custom object is not properly handled in RichText demo

[afester]

• Launch the RichTextDemo
• Insert an image
• Place cursor to start of paragraph
• Press Enter
  => Image is shown twice, once in the upper (new) paragraph and once in the lower paragraph.
  When using an older variant of the LinkedImageOps, it works well.

[afester]

Modifying realSubSequence in NodeSegmentOpsBase so that it creates an empty segment when the sub sequence length is 0 fixes it:

    @Override
    public SEG realSubSequence(SEG seg, int start, int end) {
        if (start == end) {
            return createEmpty();
        }
        return seg;
    }

Needs some additional testing before submitting a PR, but I think this should be a safe assumption in any case (and thus should probably be moved to subSequence in SegmentOpsBase)

[JordanMartinez]

This issue no longer arises in #590

[JordanMartinez]

Closed by #590