fix: skip a None content value in a multi-entry page join instead of rendering literal 'None'

the multi-entry content-join f-strung a dict entry's content with no None guard, so a
{'content': None} entry following a text entry produced a literal 'None' line
('hello' + None -> 'hello\nNone'). a None content now contributes nothing to the join.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_paginator/dpy_paginator.py

@@ -280,6 +280,8 @@ class DPYPaginator(Generic[PageT_co], discord.ui.View):
                         f"page key {key!r} has unexpected type {type(value).__name__}"
                     )
                 elif key == "content":
+                    if value is None:
+                        continue
                     existing = kwargs["content"]
                     kwargs["content"] = value if existing is None else f"{existing}\n{value}"
                 else: