Ignore blank alts if site alt config is enabled

If the alt for a particular service is blank, the original source is
used instead.

Example:
1. Site alts enabled in config
2. User wants wikipedia links, not wikiless
3. WHOOGLE_ALT_WIKI set to ""
4. All available alt links redirected to farside, except wikipedia

Fixes #704

app/filter.py

@@ -410,7 +410,7 @@ class Filter:
             # Replace link description
             link_desc = link_desc[0]
             for site, alt in SITE_ALTS.items():
-                if site not in link_desc:
+                if site not in link_desc or not alt:
                     continue
                 new_desc = BeautifulSoup(features='html.parser').new_tag('div')
                 new_desc.string = str(link_desc).replace(site, alt)

app/utils/results.py

@@ -128,7 +128,7 @@ def get_site_alt(link: str) -> str:
     hostname = urlparse.urlparse(link).hostname
 
     for site_key in SITE_ALTS.keys():
-        if not hostname or site_key not in hostname:
+        if not hostname or site_key not in hostname or not SITE_ALTS[site_key]:
             continue
 
         link = link.replace(hostname, SITE_ALTS[site_key])