Closes #4090

In the earlier PR #3765, I optimized VMobject.add_points_as_corners() to allocate space for VMobject.points only once, instead of calling N times in a for loop the add_line_to() method which allocated space N times, one for each point. This is really helpful for drawing and animating multiple ParametricFunctions and/or ImplicitFunctions, which tend to require the computation of a lot of points which are then interpolated smoothly.

However, I missed the edge case in which the points parameter contains 0 points, which caused the crash described in #4090. It is possible that the ImplicitFunction algorithm for finding a sample of points belonging to the curve returns a single point for one or more branches (called curves) of the full implicit curve. In that case, the following three lines in ImplicitFunction.generate_points() will fail because of that mistake:

        for curve in curves:
            self.start_new_path(curve[0])
            self.add_points_as_corners(curve[1:])

In this PR, I fix this behavior by returning earlier from VMobject.add_points_as_corners() if points is empty, in a similar way to what the original method already implicitly did (no points = no iterations inside the original for loop).

Plus, instead of returning the passed points (which is useless, because you already had the points beforehand), I made this method return self (which is consistent with many other methods and allows us to chain this method with other subsequent calls).

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested