Fix metric calculation warnings

[Lilly-May]

Changes proposed in this pull request:

• Fixed warnings in _get_relative_expression_similarity_across_genes_grid that were caused by division by zero.
• Division by zero occurs when there are either no cells in the respective grid field or when there is no overlap between cell types of spatial and sc data.
• This doesn't change the result of the metric calculation, as in those two cases, the metric values for that grid field end up being NaN before and after this PR. However, merging this PR will remove a lot of warnings and save a bit of computing time.

[LouisK92]

Added a similar code snippet to the metric for relative expr between ct pairs. For that metric we need at least 2 cell types in a grid field.