Handle empty graph data

[hou098]

If there are no matching samples the graph popup hangs with the spinning loading helix.

e.g. metagenome search for Bacteria Cyanobacteria Cyanobacteria Subgroup1 (with https://github.com/BioplatformsAustralia/bpaotu/tree/metagenome-feature-WIP-demo )

i.e. requests to /private/api/v1/taxonomy-graph-fields can return

{"graphdata": {"traits": {}, "amplicon": {}, "taxonomy": {}, "am_environment_all": [], "am_environment": {}, "am_environment_selected": {}, "am_environment_non_selected": {}, "traits_am_environment_all": [], "traits_am_environment": {}, "traits_am_environment_selected": {}, "traits_am_environment_non_selected": {}}}

and /private/api/v1/contextual-graph-fields

{"graphdata": {}}