[BUGFIX] ExtDirectApi uses an undefined variable

Method generateAPI() sets $className inside the condition but later uses
it without checking if it is empty or not. The result is a PHP warning
"Invalid argument supplied for foreach()".

Change-Id: I7fd2bf88374661a8b54023aee9fe2af5619471b0
Resolves: #46075
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/19115
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn