Treat empty inputs as non-errors in two wiki validators.

If VLength (or its subclass) gets an empty input, by default it will set
an error state. This error state now has a code param, which causes wiki
code to treat it as fatal. This change disables the empty error in two
places, when appropriate.

r2/r2/controllers/reddit_base.py

@@ -582,7 +582,7 @@ def paginated_listing(default_page_size=25, max_page_size=100, backend='sql'):
                   before=VByName('before', backend=backend),
                   count=VCount('count'),
                   target=VTarget("target"),
-                  show=VLength('show', 3))
+                  show=VLength('show', 3, empty_error=None))
         @wraps(fn)
         def new_fn(self, before, **env):
             if c.render_style == "htmllite":

r2/r2/controllers/wiki.py

@@ -299,7 +299,7 @@ class WikiApiController(WikiController):
               pageandprevious=VWikiPageRevise(('page', 'previous'), restricted=True),
               content=nop(('content')),
               page_name=VWikiPageName('page'),
-              reason=VPrintable('reason', 256))
+              reason=VPrintable('reason', 256, empty_error=None))
     @api_doc(api_section.wiki, uri='/api/wiki/edit')
     def POST_wiki_edit(self, pageandprevious, content, page_name, reason):
         page, previous = pageandprevious