fix: DH-14439 Fix QueryMonitor breaking on "null" in default search filter

[vbabich]

Allow null in match filters on string columns, treat it as a regular string value.

Testing steps:

Run this query:

from deephaven import empty_table
a = [None, 'Null', 'null - starts with', 'ends with null', 'null', 'ABC']
test_table = empty_table(6).update(formulas=["X = (String)a[i]"])

Filter using quick filters treating null as a string:

• *null, =*null, !=*null - ends with, does not end with
• null*, =null*, !=null* - starts with, does not start with
• ~null, !~null - contains/does not contain

Special cases treating null as a null value:

• =null, !=null