`(N/A)` label appears in the groupby list even when there are 0 N/A elements

## Description

Noted during a demo by Majid. The `(N/A)` placeholder value appeared even though there were 0 N/A values. It seems to occur when the group by and color by features are different.

## Expected Behavior

- `(N/A)` should not appear when every row in the CSV has an element

## Reproduction

https://cfe.allencell.org/?cellSelectedFor3D=2819&colorBy=Gene&dataset=csv&csvUrl=https%253A%252F%252Fvast-files.int.allencell.org%252Frep_learn%252FMF%252FOV%252Ffeatures_subset_data_voxel%252Fres%252Fembeddings_val_sub_structure.csv&plotByOnX=tsne_2&plotByOnY=tsne_1&groupBy=Cell%20Line&selectedPoint%5B0%5D=3006&selectedPoint%5B1%5D=6086&selectedPoint%5B2%5D=16659&selectedPoint%5B3%5D=2819

## Environment

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