[FIX] Fix error on link create

[ales-erjavec]

When in open anchors mode dragging a outgoing link back onto the node itself would raise an:

ValueError: min() arg is an empty sequence

in AnchorItem.signalAtPos

---------------------------- ValueError Exception -----------------------------
Traceback (most recent call last):
  File "/home/ales/devel/orange-canvas-core/orangecanvas/canvas/scene.py", line 848, in mouseReleaseEvent
    self.user_interaction_handler.mouseReleaseEvent(event):
  File "/home/ales/devel/orange-canvas-core/orangecanvas/document/interactions.py", line 610, in mouseReleaseEvent
    sink_signal = item.inputAnchorItem.signalAtPos(
  File "/home/ales/devel/orange-canvas-core/orangecanvas/canvas/items/nodeitem.py", line 1024, in signalAtPos
    return min(signalsToFind, key=signalLengthToPos)
ValueError: min() arg is an empty sequence
-------------------------------------------------------------------------------

[codecov-commenter]

Codecov Report

Merging #220 (f030783) into master (f8f944c) will increase coverage by 0.00%. The diff coverage is 25.00%.

@@           Coverage Diff           @@
##           master     #220   +/-   ##
=======================================
  Coverage   74.03%   74.03%           
=======================================
  Files          95       95           
  Lines       20267    20272    +5     
=======================================
+ Hits        15004    15008    +4     
- Misses       5263     5264    +1     

Impacted Files	Coverage Δ
orangecanvas/document/interactions.py	41.74% <25.00%> (-0.11%)	:arrow_down:
orangecanvas/utils/overlay.py	79.95% <0.00%> (+0.40%)	:arrow_up:
orangecanvas/gui/dock.py	79.84% <0.00%> (+0.77%)	:arrow_up:

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