chore(context): test context initialization and handler logic (#4087)

* enhance code imported by #3413

if it needs to check if the handler is nil, tie c.index shall
always ++

* test: refactor test context initialization and handler logic

- Remove an empty line in `TestContextInitQueryCache`
- Add `TestContextNext` function with tests for `Next` method behavior with no handlers, one handler, and multiple handlers

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

---------

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-authored-by: zjj <zhong2plus@gmail.com>

context.go

@@ -186,10 +186,9 @@ func (c *Context) FullPath() string {
 func (c *Context) Next() {
 	c.index++
 	for c.index < int8(len(c.handlers)) {
-		if c.handlers[c.index] == nil {
-			continue
+		if c.handlers[c.index] != nil {
+			c.handlers[c.index](c)
 		}
-		c.handlers[c.index](c)
 		c.index++
 	}
 }