make nimweb compile again

lib/impure/re.nim

@@ -613,7 +613,7 @@ when isMainModule:
     doAssert false
 
   if "abc" =~ re"(cba)?.*":
-    doAssert matches[0] == nil
+    doAssert matches[0] == ""
   else: doAssert false
 
   if "abc" =~ re"().*":

tools/website.tmpl

@@ -203,7 +203,7 @@ runForever()
 #  if currentTab == "index":
   <script src="${rootDir}assets/index.js"></script>
 # end if
-#  if c.gaId != nil:
+#  if c.gaId.len != 0:
   <script>
     (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
     (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),