init: allow null value for host_refs for bbmap in config

workflow/rules/init.smk

@@ -29,7 +29,7 @@ if config["data"]["metat"]["sr"]["r1"] and config["data"]["metat"]["sr"]["r2"]:
 NO_BASECALLING = config["data"]["metag"]["ont"]["fastq"] and os.path.exists(config["data"]["metag"]["ont"]["fastq"])
 
 # Whether host contamination has to be removed - only if host references are given
-NO_HOST_FILTERING = len(config["bbmap"]["host_refs"].keys()) == 0
+NO_HOST_FILTERING = config["bbmap"]["host_refs"] is None or len(config["bbmap"]["host_refs"].keys()) == 0
 
 # Assemblers and read types
 META_TYPES = ["metag"]