Fix DE result filenames to correctly handle N/A-type metadata values (SCP-4436)

[jlchang]

This PR modifies DE to only convert empty cells to --Unspecified-- and leaves NA strings as-is so they are faithful to values in the metadata file.

Background:
Currently N/A-like strings as annotation labels currently will not display DE results. Pandas detects empty cells as numeric NaN which is problematic for group annotations so DE converts those values to --Unspecified-- as the DE annotation label. Unfortunately, pandas also converts a set of default NaN recognized values (like N/A). Since we construct the file name for data retrieval from annotationList values - if the value is one of the "pandas default NaN recognized values", the value in the filename is --Unspecified-- instead of N/A so the file is not found. ().

Manual test:
activate the scp-ingest-pipeline repo virtualenv
then from the scripts directory of the scp-ingest-pipeline repo, perform this setup:

source ../scripts/setup_mongo_dev.sh
unset BARD_HOST_URL

Run the DE job from the ingest directory of the scp-ingest-pipeline repo:
python ingest_pipeline.py --study-id addedfeed000000000000000 --study-file-id dec0dedfeed1111111111111 differential_expression --annotation-name cell_type__ontology_label --annotation-type group --annotation-scope study --matrix-file-path ../tests/data/differential_expression/de_dense_matrix.tsv --matrix-file-type dense --annotation-file ../tests/data/differential_expression/de_dense_metadata_na.txt --cluster-file ../tests/data/differential_expression/de_dense_cluster.tsv --cluster-name de_na --study-accession SCPna --differential-expression

confirm that the job runs successfully and the output files have the expected filenames:

de_na--cell_type__ontology_label--N_A--study--wilcoxon.tsv
de_na--cell_type__ontology_label--NaN--study--wilcoxon.tsv
de_na--cell_type__ontology_label--__Unspecified__--study--wilcoxon.tsv
de_na--cell_type__ontology_label--cholinergic_neuron--study--wilcoxon.tsv
de_na--cell_type__ontology_label--cranial_somatomotor_neuron--study--wilcoxon.tsv
de_na--cell_type__ontology_label--null--study--wilcoxon.tsv
de_na--cell_type__ontology_label--pyramidal_neuron--study--wilcoxon.tsv
de_na--cell_type__ontology_label--somatomotor_neuron--study--wilcoxon.tsv
de_na--cell_type__ontology_label--sympathetic_cholinergic_neuron--study--wilcoxon.tsv

This PR satisfies SCP-4436.