veb: add ctx.no_content() + prevent content-type being set if the mime type is empty (#23425)

Author: louis77

vlib/veb/README.md

@@ -758,6 +758,8 @@ ctx.json(User{
 	name: 'test'
 	age: 20
 })
+// send response HTTP_NO_CONTENT (204) without a content-type and body
+ctx.no_content()
 ```
 
 #### Sending files

vlib/veb/context.v

@@ -99,7 +99,9 @@ pub fn (mut ctx Context) send_response_to_client(mimetype string, response strin
 
 	// set Content-Type and Content-Length headers
 	mut custom_mimetype := if ctx.content_type.len == 0 { mimetype } else { ctx.content_type }
-	ctx.res.header.set(.content_type, custom_mimetype)
+	if custom_mimetype != '' {
+		ctx.res.header.set(.content_type, custom_mimetype)
+	}
 	if ctx.res.body != '' {
 		ctx.res.header.set(.content_length, ctx.res.body.len.str())
 	}
@@ -225,6 +227,12 @@ pub fn (mut ctx Context) server_error(msg string) Result {
 	return ctx.send_response_to_client('text/plain', msg)
 }
 
+// send a 204 No Content response without body and content-type
+pub fn (mut ctx Context) no_content() Result {
+	ctx.res.set_status(.no_content)
+	return ctx.send_response_to_client('', '')
+}
+
 @[params]
 pub struct RedirectParams {
 pub: