Geometry_Engine: null checks for methods with default result

There are methods for which some types of input are more of less not applicable. (i.e. `Area(Line)`, `DiscontinuityPoints(Circle)`). Those method always return default results like empty list or value `0`.

It follows that even if they receive a null input they still return some result without logging any error.
Maybe it's worth adding some null check to help detecting errors.

Examples:

@pawelbaran @IsakNaslundBh @al-fisher @FraserGreenroyd

Start by locating the Geometry_Engine methods represented by Area(Line) and DiscontinuityPoints(Circle), then inspect how null inputs currently produce default results. Determine the intended null-check and error-logging behavior for these methods, and verify that applicable inputs still return their existing results.

Written by the indexing model from the issue text.