`addEditedAlleles()` fails when the guide sequence is not editable

[RussBainer]

Hi JP and Team, really enjoying this set of tools.

I've run into an error from the addEditingWeights() function that occurs when the targeted sequence doesn't have any nucleotides with values greater than zero in the base editor weight matrix.

I'm not sure how you'd want to address circumstances like this in terms of the functionality of the code, so I thought I'd raise it as an issue instead of a PR. I think you can resolve it on line 220 of addEditedAlleles.R by checking length(nNucs) > 0. For example:

> crisprDesign:::.getExtendedSequences(gs, start = editingWindow[1], 
+                                      end = editingWindow[2])
spacer_1160 
   "GGGTGGTTGGGGT" 
> addEditedAlleles(gs, baseEditor = abe, txTable = txtable, editingWindow = win)
[addEditedAlleles] Obtaining edited alleles at each gRNA target site.
Error in -out$score : invalid argument to unary operator

[Jfortin1]

Hi @RussBainer, taking a look at it right now

[Jfortin1]

Pushed a fix; let me know if that works

[RussBainer]

Sorry @Jfortin1 ! I just realized that my github notifications were going to my old work email.

This looks like a good fix. Thanks!