[BUGFIX] Render ext icon in EM only if available

Even if the ext icon is required, it can happen that it is still not
there. Currently this leads to an ugly output which can be easily
fixed by just checking if the icon is there.

Change-Id: I207a93bb796510fba6096085a86f1dd832100e06
Resolves: #54838
Releases: 6.2
Reviewed-on: https://review.typo3.org/26693
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters