Allow empty params.mainSections

Updates #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