Geometry_Engine: provide handling of null inputs

#### Broken rules:

Follow up to #1913 - starts from [that comment](https://github.com/BHoM/BHoM_Engine/pull/1913#pullrequestreview-461493946)
Having resolved `NotImplementedException`s next step should be to handle null inputs at interfaces. At this moment if any interface gets a null input it won't pass it to any method and throw an misleading ambiguous call exception instead.

#### Suggestions to restore compliance:

- Change fallback methods input type to `object` to catch dynamically resolved `null`s - [turned out](https://github.com/BHoM/BHoM_Engine/pull/1913#issuecomment-670428183) that dynamic interfaces will not pass `null`s.
- Add NullCheck method to the interfaces and/or create another type of fallback method.

Start by reading issue #1913 and the linked pull-request review and comment, then trace the Geometry_Engine interfaces and their fallback methods. Determine how null inputs currently produce the ambiguous call exception; done means null inputs are handled at the interfaces without that misleading exception.

Written by the indexing model from the issue text.