httpClient: prevent empty user-agent in header (#8411)

Author: d-a-v

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -929,8 +929,10 @@ bool HTTPClient::sendHeader(const char * type)
  header += ':';
  header += String(_port);
  }
- header += F("\r\nUser-Agent: ");
- header += _userAgent;
+ if (_userAgent.length()) {
+ header += F("\r\nUser-Agent: ");
+ header += _userAgent;
+ }
 
  if (!_useHTTP10) {
  header += F("\r\nAccept-Encoding: identity;q=1,chunked;q=0.1,*;q=0");