This happens because $cOTLdata is not null but the index 'char_data' does not exist. Previous versions of PHP may have been less strict on such mistakes and silently swallowed the error / notice while 7.4 does not do this anymore.
To check whether the index exists or not you can use isset():
isset($cOTLdata['char_data'])
Which means the line should look something like this:
$len = isset($cOTLdata['char_data']) ? count($cOTLdata['char_data']) : 0;
Note I switched the then and else cases of the ternary operator since === null is essentially what isset already does (but in the positive case).
