Make "href" a mandatory argument in Xml.add_link()

The link target must always be specified. Only the its display in the text is optional.

docs/xml-class.rst

@@ -103,7 +103,7 @@ There is no need to ever directly construct an :ref:`Xml` object: after creating
        :arg width: if provided, either an absolute (int) value, or a percentage string like "30%". A percentage value refers to the width of the specified ``where`` rectangle in :meth:`Story.place`. If this value is provided and ``height`` is omitted, the image will be included keeping its aspect ratio.
        :arg height: if provided, either an absolute (int) value, or a percentage string like "30%". A percentage value refers to the height of the specified ``where`` rectangle in :meth:`Story.place`. If this value is provided and ``width`` is omitted, the image's aspect ratio will be honored.
 
-    .. method:: add_link(href=None, text=None)
+    .. method:: add_link(href, text=None)
 
        Add an :htmlTag:`a` tag - inline element, treated like text.
 

fitz/fitz.i

@@ -13122,17 +13122,13 @@ struct Xml
             self.append_child(child)
             return child
 
-        def add_link(self, href=None, text=None):
+        def add_link(self, href, text=None):
             """Add a hyperlink ("a" tag)"""
             child = self.create_element("a")
-            if not isinstance(href, str):
-                href = text
             if not isinstance(text, str):
                 text = href
-            if href:
-                child.set_attribute("href", href)
-            if text:
-               child.append_child(self.create_text_node(text)) 
+            child.set_attribute("href", href)
+            child.append_child(self.create_text_node(text)) 
             prev = self.span_bottom()
             if prev == None:
                 prev = self