fix(context): check handler is nil (#3413)

* fixed #3404 2022-11-23

* up 2022-11-23

* refactor: refactor context handling and nil checks

- Refactor nil checks to improve readability in `context.go`
- Modify the control flow in `HandlerNames` and `Next` methods to continue on nil values before appending or invoking handlers in `context.go`

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

* test: refactor context_test.go for clarity and efficiency

- Insert a `nil` value into the `HandlersChain` array in `context_test.go`
- Remove empty test functions in `context_test.go`

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

---------

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

context.go

@@ -152,6 +152,9 @@ func (c *Context) HandlerName() string {
 func (c *Context) HandlerNames() []string {
 	hn := make([]string, 0, len(c.handlers))
 	for _, val := range c.handlers {
+		if val == nil {
+			continue
+		}
 		hn = append(hn, nameOfFunction(val))
 	}
 	return hn
@@ -182,6 +185,9 @@ 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
+		}
 		c.handlers[c.index](c)
 		c.index++
 	}