[BUGFIX] IconFactory check arrays for key

This reduces PHP notices, as we check arrays before acessing them.

Resolves: #78992
Releases: master, 7.6
Change-Id: Ib0fc72be9e7957a27990d60dde844068ca9e24d1
Reviewed-on: https://review.typo3.org/50971


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>