[BUGFIX] Clean up inconsistent data from import

When importing a T3D or XML file the loaded data has to get cleansed
first. The "extensionDependencies" array for example may contain an
empty string which will cause a non-correct warning message in the
importers warning tab.

A new method "cleanupImportData" takes care to bring import data into
an acceptable format.

Resolves: #72761
Releases: master, 7.6
Change-Id: I79dcccb92f453902b4b83be243d2f89af711246c
Reviewed-on: https://review.typo3.org/45993


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>