Allow empty params.mainSections

Updates gohugoio#10953

config/allconfig/allconfig.go

@@ -367,7 +367,7 @@ type ConfigCompiled struct {
 func (c *ConfigCompiled) SetMainSectionsIfNotSet(sections []string) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
-	if len(c.MainSections) > 0 {
+	if c.MainSections != nil {
 		return
 	}
 	c.MainSections = sections

config/allconfig/alldecoders.go

@@ -177,6 +177,9 @@ var allDecoderSetups = map[string]decodeWeight{
 			// Before Hugo 0.112.0 this was configured via site Params.
 			if mainSections, found := p.c.Params["mainsections"]; found {
 				p.c.MainSections = types.ToStringSlicePreserveString(mainSections)
+				if p.c.MainSections == nil {
+					p.c.MainSections = []string{}
+				}
 			}
 
 			return nil

hugolib/config_test.go

@@ -871,3 +871,29 @@ Param: svParamValue
 `)
 
 }
+
+func TestConfigEmptyMainSections(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.yml --
+params:
+  mainSections:
+-- content/mysection/_index.md --
+-- content/mysection/mycontent.md --
+-- layouts/index.html --
+mainSections: {{ site.Params.mainSections }}
+
+`
+	b := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).Build()
+
+	b.AssertFileContent("public/index.html", `
+mainSections: []
+`)
+
+}