vlib: refactor empty string checks to use s == '' or s != '', instead of s.len == 0 (#21300)

vlib/net/http/file/folder_index.v

@@ -60,7 +60,7 @@ fn write_page_table(mut sb strings.Builder, uri_path string, requested_file_path
 	sb.writeln('<th align="left" style="width: 200px">Last modified</th>')
 	sb.writeln('<th align="left">Name</th>')
 	sb.writeln('</tr>')
-	if uri_path.len == 0 {
+	if uri_path == '' {
 		sb.writeln('<tr>')
 		sb.writeln('<td>---</td>')
 		sb.writeln('<td>---</td>')

vlib/net/http/http_test.v

@@ -17,7 +17,7 @@ fn test_http_get_from_vlang_utc_now() {
 		println('Test getting current time from ${url} by http.get')
 		res := http.get(url) or { panic(err) }
 		assert res.status() == .ok
-		assert res.body.len > 0
+		assert res.body != ''
 		assert res.body.int() > 1566403696
 		println('Current time is: ${res.body.int()}')
 	}
@@ -39,7 +39,7 @@ fn test_public_servers() {
 		println('Testing http.get on public url: ${url} ')
 		res := http.get(url) or { panic(err) }
 		assert res.status() == .ok
-		assert res.body.len > 0
+		assert res.body != ''
 	}
 }
 
@@ -51,6 +51,6 @@ fn test_relative_redirects() {
 	} // tempfix periodic: httpbin relative redirects are broken
 	res := http.get('https://httpbin.org/relative-redirect/3?abc=xyz') or { panic(err) }
 	assert res.status() == .ok
-	assert res.body.len > 0
+	assert res.body != ''
 	assert res.body.contains('"abc": "xyz"')
 }

vlib/net/http/response.v

@@ -131,7 +131,7 @@ pub fn new_response(conf ResponseConfig) Response {
 		body: conf.body
 		header: conf.header
 	}
-	if resp.body.len > 0 && !resp.header.contains(.content_length) {
+	if resp.body != '' && !resp.header.contains(.content_length) {
 		resp.header.add(.content_length, resp.body.len.str())
 	}
 	resp.set_status(conf.status)