[BUGFIX] Do not add error in TypoScriptParser if modifier returns null

Currently, the TypoScriptParser adds an error when
the result of a modifier is null. This is, because
the line is not set to a new value in such a case
and the following switch statement is not expecting
a line with a value modifier.

This has been fixed by skipping further checks on
the value, if the modifier returns null.

Resolves: #94109
Releases: master, 11.5
Change-Id: I28ae1570a5b0b4fc5ad667bacd86a17234aa1c60
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