Handle empty and scalar cases in string conversion

Fixed string conversion for empty objects to return empty string
Add extra step in ControlledTerm to ensure scalar string "" is treated as empty

Author: ehennestad

code/internal/+openminds/+abstract/ControlledTerm.m

@@ -50,6 +50,10 @@
                 propValues.id (1,1) string
             end
 
+            if isstring(instanceSpec) && isscalar(instanceSpec) && instanceSpec == ""
+                instanceSpec = string.empty;
+            end
+
             if ~isempty(instanceSpec)
                 if ischar(instanceSpec)
                     instanceSpec = string(instanceSpec);

code/internal/+openminds/+internal/+mixin/CustomInstanceDisplay.m

@@ -30,10 +30,12 @@
         end
 
         function str = string(obj)
-            if numel(obj) > 1
-                str = strjoin( arrayfun(@(x) string(x.DisplayString), obj ), '; ');
-            else
+            if isempty(obj)
+                str = string.empty;
+            elseif isscalar(obj)
                 str = string( obj.DisplayString );
+            else
+                str = strjoin( arrayfun(@(x) string(x.DisplayString), obj ), '; ');
             end
         end