Handle empty and scalar cases in string conversion (#77)

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