[BUGFIX] Warning in getCompressedTCarray due to missing extTables.php

Since 6.0, constant TYPO3_extTableDef_script falls back to string
'extTables.php' if it is not defined otherwise. This might trigger
warnings for unclean code that used this constant before if the file
does not exist.

Fixes: #47002
Related: #44331
Related: #45122
Releases: 6.0
Change-Id: Id4e7f492f262735e965c70df0b770c62a6923773
Reviewed-on: https://review.typo3.org/19706
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
[BUGFIX] Write config to extTables destroys HTML output

If you enter HTML to update a field in e.g. TCA to write the value into
extTables.php, then the HTML will be printed into the configuration
module.
htmlspecialchar the output.

Fixes: #46999
Releases: 6.1,6.0,4.7,4.5
Change-Id: I390b4252316b8bdf01e5bbcc5a8b33833bdf73e8
Reviewed-on: https://review.typo3.org/19707
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn