[BUGFIX] Set correct default values in roundtrip mode

The class builder now sets correct default values for existing properties.
Since there might be cases where it makes a difference if a class property
is NULL or an empty string or 0 (for numeric type) this patch adds a
configuration flag setDefaultValuesForClassProperties which can be set
to false to disable this behaviour (tbd)

Resolves: #58164
Releases: 6.2
Change-Id: I2a00346846470ab8b368da8f29e9dfb8d031d1d0
Reviewed-on: https://review.typo3.org/29642
Reviewed-by: Steffen Müller
Tested-by: Steffen Müller
Reviewed-by: Nico de Haen
Tested-by: Nico de Haen