fix: always make an edge label, even when empty
Processing certain graphs would result in failures in the spline routing code:
Error: in routesplines, illegal values of prev 1 and next 1, line 441
Error: in routesplines, illegal values of prev 1 and next 1, line 441
Error: lost 376 256 edge
Error: lost 256 376 edge
Instead of bypassing the label construction code in the case of an edge label being empty, the label is now unconditionally constructed. The label itself should still not appear as it has no content.
It is not clear to me why this change fixes the problem. The change was arrived at by typoing some unrelated development work and observing that the unintended edits surprisingly fixed the #2368 (closed) test case. The test case itself has no empty labels, so one would expect this change to be a no-op for it. But apparently it is not.
Gitlab: #1887, #2436, fixes #2368 (closed)
Reported-by: kayoch1n