[type/__uci] Allow omission of --value parameter if --state absent

cdist/conf/type/__uci/explorer/state

@@ -40,6 +40,14 @@ values_is=$(uci -s -N -d "${RS}" get "${option}" 2>/dev/null) || {
 	exit 0
 }
 
+if test -f "${__object:?}/parameter/value"
+then
+	should_file="${__object:?}/parameter/value"
+else
+	should_file='/dev/null'
+fi
+
+
 # strip off trailing newline
 printf '%s' "${values_is}" \
 | awk '
@@ -86,4 +94,4 @@ END {
 
 	print state
 }
-' "${__object:?}/parameter/value" RS="${RS}" -
+' "${should_file}" RS="${RS}" -

cdist/conf/type/__uci/man.rst

@@ -20,6 +20,7 @@ REQUIRED PARAMETERS
 -------------------
 value
     The value to be set. Can be used multiple times.
+    This parameter is allowed to be omitted if `--state` is `absent`.
 
     Due to the way cdist handles arguments, values **must not** contain newline
     characters.

cdist/conf/type/__uci/manifest

@@ -43,6 +43,11 @@ changes_required=false
 case ${state_should}
 in
 	(present)
+		test -s "${__object:?}/parameter/value" || {
+			echo 'The parameter --value is required.' >&2
+			exit 1
+		}
+
 		# NOTE: order is ignored so rearranged is also fine.
 		in_list "${state_is}" 'present' 'rearranged' || changes_required=true
 		;;