R1:66192a5b832c

Fix "Undefined index: icon" when visiting Search Servers using MySQL

Summary:
Fix "Undefined index: icon" when visiting Search Servers using MySQL

NOTE: This patch just fixes the exception at my best but this section probably deserves more improvement to show a better default.

Closes T15155

Test Plan:
- use the default Search Server configuration (that is MySQL)
- open the page Search Servers (/config/cluster/search/)
- verify that it does not explode anymore but it displays something unuseful…