Fix panic when using chain middleware.

pkg/responsemodifiers/response_modifier.go

@@ -45,7 +45,10 @@ func (f *Builder) Build(ctx context.Context, names []string) func(*http.Response
 			for _, name := range conf.Chain.Middlewares {
 				qualifiedNames = append(qualifiedNames, provider.GetQualifiedName(chainCtx, name))
 			}
-			modifiers = append(modifiers, f.Build(ctx, qualifiedNames))
+
+			if rm := f.Build(ctx, qualifiedNames); rm != nil {
+				modifiers = append(modifiers, rm)
+			}
 		}
 	}
 

pkg/responsemodifiers/response_modifier_test.go

@@ -169,6 +169,21 @@ func TestBuilderBuild(t *testing.T) {
 			},
 			assertResponse: func(t *testing.T, resp *http.Response) {},
 		},
+
+		{
+			desc:          "chain without headers",
+			middlewares:   []string{"chain"},
+			buildResponse: stubResponse,
+			conf: map[string]*dynamic.Middleware{
+				"foo": {IPWhiteList: &dynamic.IPWhiteList{}},
+				"chain": {
+					Chain: &dynamic.Chain{
+						Middlewares: []string{"foo"},
+					},
+				},
+			},
+			assertResponse: func(t *testing.T, resp *http.Response) {},
+		},
 	}
 
 	for _, test := range testCases {