phylo: Make auspice_name config optional

If `auspice_name` param is not provided, then the workflow will use
the required `build_name` as the default Auspice filename.

Ensures that we are following the Nextstrain Snakemake style guide for
configs¹ by not accessing configs with `config.get(key)`.

¹ https://docs.nextstrain.org/en/latest/reference/snakemake-style-guide.html#access-config-values-appropriately

phylogenetic/Snakefile

@@ -16,14 +16,12 @@ if not config:
 
 
 build_dir = "results"
-
-
 auspice_dir = "auspice"
 
 prefix = config.get("auspice_prefix", None)
 AUSPICE_PREFIX = ("trial_" + prefix + "_") if prefix is not None else ""
-AUSPICE_FILENAME = AUSPICE_PREFIX + config.get("auspice_name")
-
+# Defaults to the `build_name` if no `auspice_name` is provided in the config
+AUSPICE_FILENAME = AUSPICE_PREFIX + config.get("auspice_name", config["build_name"])
 
 rule all:
     input: