Only use sqlapi configuration "host" if "domain" is undefined

We'll consider an empty string domain as valid (it's actually used
for testsuite).

lib/cartodb/server_options.js

@@ -9,7 +9,7 @@ var   _          = require('underscore')
   ;
 
 // This is for backward compatibility with 1.3.3
-if ( ! global.environment.sqlapi.domain ) {
+if ( _.isUndefined(global.environment.sqlapi.domain) ) {
   global.environment.sqlapi.domain = global.environment.sqlapi.host;
 }