grammar: Parse columns with explicit NULL (un-)constraint correctly

Table definitions like these wouldn't work before:
CREATE TABLE test(
    a int NOT NULL,    -- This worked fine
    b int NULL         -- This didn't but does now.
);

See issue #741.

src/sqlitetypes.cpp

@@ -739,6 +739,11 @@ void CreateTableWalker::parsecolumn(Table& table, antlr::RefAST c)
             notnull = true;
         }
         break;
+        case sqlite3TokenTypes::NULL_T:
+        {
+            notnull = false;
+        }
+        break;
         case sqlite3TokenTypes::CHECK:
         {
             con = con->getNextSibling(); //LPAREN