fix(edges): prevent the attrs from overwriting by empty string

diagrams/__init__.py

@@ -445,8 +445,11 @@ class Edge:
         self._attrs["xlabel"] = label if label else xlabel
         self._attrs["color"] = color
         self._attrs["style"] = style
+        if fontcolor:
             self._attrs["fontcolor"] = fontcolor
+        if fontname:
             self._attrs["fontname"] = fontname
+        if fontsize:
             self._attrs["fontsize"] = fontsize
 
     def __sub__(self, other: Union["Node", "Edge", List["Node"]]):