[BUGFIX] RTE not loading in IE when content css file does not exist

Solution: Fallback to the default contentCSS file when the file
specified in Page TS Config does not exist or is of zero size.

Change-Id: I18fe77f5fad13a244e4dc8570bbdf940b0721fcc
Resolves: #29899
Releases: 4.5, 4.6, 4.7
Reviewed-on: http://review.typo3.org/6650
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland