Fix corner case issue in BlendExpression

[egeulgen]

This attempts to fix the issue when one of the features provided to BlendExpression is constant, min-max normalization returns NaN, yielding unusable output data.

This issue was discovered when we used blend with split.by:

FeaturePlot(seu_obj, features = c("metadata", "GeneSymbol"), blend = TRUE, split.by = "group")

# Error in `palette()`:
# ! Insufficient values in manual scale. 1 needed but only 0 provided.
# Run `rlang::last_trace()` to see where the error occurred.