Merge branch '416-modulecmd-manpath-must-be-initialised-if-not-already-defined' into 'master'

Resolve "modulecmd: MANPATH must be initialised if not already defined"

Closes #416

See merge request Pmodules/src!448

Pmodules/modulecmd.bash.in

@@ -82,13 +82,14 @@ declare -A Subcommands=()
 declare -A Options=()
 declare -A Help=()
 
-# these variables must exist
+# these variables must exist. Under some conditions Lmod unset them.
 if [[ ! -v LOADEDMODULES ]] || [[ ! -v _LMFILES_ ]]; then
 	LOADEDMODULES=''
 	_LMFILES_=''
 fi
 [[ -v MODULEPATH ]]	   || MODULEPATH=''
 [[ -v PMODULES_ENV ]]      || PMODULES_ENV=''
+[[ -v MANPATH ]]           || MANPATH=''
 
 # initialize help text of 'module --version'
 Help['version']="