Fix lineTo to do nothing instead of raising error when 0 length

[endikallanomatxin]

Before the change it raised an error when startPoint and endPoint were equal

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.83%. Comparing base (33f72d2) to head (e3d6907).

Files	Patch %	Lines
cadquery/cq.py	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1606 +/- ## ========================================== - Coverage 94.86% 94.83% -0.04% ========================================== Files 28 28 Lines 6231 6233 +2 Branches 1264 1265 +1 ========================================== Hits 5911 5911 - Misses 193 194 +1 - Partials 127 128 +1 ```

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