Bypass rule tree error

[ppcad]

Logprep matches filter keys on event values if the environment variable LOGPREP_BYPASS_RULE_TREE is set and a specific value is being filtered.
This raises a TypeError while parsing the filter expression, since it tries to get the key for a event value, which is a string:

logprep/filter/expression/filter_expression.py", line 90, in _get_value   
    current = current[item]
TypeError: string indices must be integers

Expected behavior
Rules never use event values to match filter keys.

Steps to reproduce

1. Start Logprep with the environment variable LOGPREP_BYPASS_RULE_TREE being set
2. Create any rule with a filter like key1.key2: value (just key1.key2 without a value won't cause problems)
3. Send an event like {"key1": "key2 is not a key"}

The rule will match and a TypeError will be raised.

Environment

Logprep version: 56cf412
Python version: 3.11