Fix assignment to entry in nil map

Fixes #13853

hugolib/cascade_test.go

@@ -965,3 +965,27 @@ All.
 
 	b.AssertLogContains("! WARN")
 }
+
+func TestCascadeNilMapIssue13853(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- content/test/_index.md --
+---
+title: Test
+cascade:
+- build:
+    list: local
+  target:
+    path: '{/test/**}'
+- params:
+    title: 'Test page'
+  target:
+    path: '{/test/**}'
+---
+`
+
+	// Just verify that it does not panic.
+	_ = Test(t, files)
+}

resources/page/page_matcher.go

@@ -177,6 +177,9 @@ func mapToPageMatcherParamsConfig(m map[string]any) (PageMatcherParamsConfig, er
 	if pcfg.Fields == nil {
 		pcfg.Fields = make(maps.Params)
 	}
+	if pcfg.Params == nil {
+		pcfg.Params = make(maps.Params)
+	}
 	for k, v := range m {
 		switch strings.ToLower(k) {
 		case "_target", "target":
@@ -186,9 +189,6 @@ func mapToPageMatcherParamsConfig(m map[string]any) (PageMatcherParamsConfig, er
 			}
 			pcfg.Target = target
 		case "params":
-			if pcfg.Params == nil {
-				pcfg.Params = make(maps.Params)
-			}
 			params := maps.ToStringMap(v)
 			for k, v := range params {
 				if _, found := pcfg.Params[k]; !found {
@@ -196,7 +196,6 @@ func mapToPageMatcherParamsConfig(m map[string]any) (PageMatcherParamsConfig, er
 				}
 			}
 		default:
-
 			pcfg.Fields[k] = v
 		}
 	}

resources/page/page_matcher_test.go

@@ -89,6 +89,7 @@ func TestPageMatcher(t *testing.T) {
 			return v
 		}
 		c.Assert(fn(map[string]any{"_target": map[string]any{"kind": "page"}, "foo": "bar"}), qt.DeepEquals, PageMatcherParamsConfig{
+			Params: maps.Params{},
 			Fields: maps.Params{
 				"foo": "bar",
 			},