Append '/' to an URL with empty path in HTTPClient::begin (#5634)

* Add an error check for empty path of an URL

* Append '/' to an URL with empty path instead of returning an error

libraries/HTTPClient/src/HTTPClient.cpp

@@ -261,6 +261,10 @@ bool HTTPClient::beginInternal(String url, const char* expectedProtocol)
     url.remove(0, (index + 3)); // remove http:// or https://
 
     index = url.indexOf('/');
+    if (index == -1) {
+        index = url.length();
+        url += '/';
+    }
     String host = url.substring(0, index);
     url.remove(0, index); // remove host part