Let quantity.Discrete/subs handle empty quantities

+quantity/Discrete.m

@@ -1042,22 +1042,23 @@ classdef  (InferiorClasses = {?quantity.Symbolic}) Discrete ...
 			[newValues, oldDomainName] = quantity.Discrete.subsParser(oldDomainName, varargin);
 			
 			solution = copy(obj);
-			for it = 1 : numel(oldDomainName)
-				if isstring(newValues{it})
-					% if newValues{it} is a string, it is converted in a domain, to use the same
-					% implementation as for quantity.Domain-objects in newValues.
-					newValues{it} = solution(1).domain.find(oldDomainName(it)).rename(newValues{it}, oldDomainName(it));
-				end
-
-				if isnumeric(newValues{it})
-					solution = solution.subsNumeric(oldDomainName(it), newValues{it});
-
-				else
-					solution = solution.subsDomain(oldDomainName(it), newValues{it});
-
-				end % if-else
-			end % for it = 1 : numel(oldDomainName)
-			
+			if ~isempty(solution)
+				for it = 1 : numel(oldDomainName)
+					if isstring(newValues{it})
+						% if newValues{it} is a string, it is converted in a domain, to use the same
+						% implementation as for quantity.Domain-objects in newValues.
+						newValues{it} = solution(1).domain.find(oldDomainName(it)).rename(newValues{it}, oldDomainName(it));
+					end
+					
+					if isnumeric(newValues{it})
+						solution = solution.subsNumeric(oldDomainName(it), newValues{it});
+						
+					else
+						solution = solution.subsDomain(oldDomainName(it), newValues{it});
+						
+					end % if-else
+				end % for it = 1 : numel(oldDomainName)
+			end % if - isempty
 		end % subs()
 		
 		function solution = subsNumeric(obj, oldDomainName, value)