Fix #any? and #none? on empty QueryBuilders

It was always adding a `WHERE` clause to the query, regardless of
whether there were any conditions.

Author: jgaskins

spec/interro_spec.cr

@@ -663,6 +663,9 @@ describe Interro do
       matching.none?.should eq false
       empty.any?.should eq false
       empty.none?.should eq true
+      # Check whether we can do it on an empty QueryBuilder
+      UserQuery.new.any?.should eq true
+      UserQuery.new.none?.should eq false
     end
 
     describe "transactions" do

src/query_builder.cr

@@ -508,11 +508,15 @@ module Interro
       sql = String.build do |str|
         str << "SELECT 1 AS one"
         str << " FROM " << sql_table_name
+
         if join = join_clause
           join.each(&.to_sql(str))
         end
 
-        str << " WHERE " << @where_clause.try(&.to_sql)
+        if where = where_clause
+          str << " WHERE " << where.to_sql
+        end
+
         str << " LIMIT 1"
       end