Fix LookML filter-expression to SQL conversion

The LookML adapter treated Looker filter values as opaque strings, so
anything beyond simple comparisons produced wrong or invalid SQL:
- date/numeric ranges and intervals ([1,10], "5 to 10") became string
  equality or malformed IN lists (matched zero rows)
- "not X" / negation -> literal "= 'not X'"
- EMPTY -> omitted the NULL case
- single quotes were not escaped (broken SQL / injection risk)

Rewrite _convert_lookml_filter_to_sql to implement the representable parts
of Looker's filter expression language (numeric ranges, interval brackets,
NOT/negation, EMPTY = NULL OR '', wildcard NOT LIKE, mixed lists) and
escape quoted string literals. Untranslated date/interval expressions now
log a warning instead of silently emitting a zero-row equality.

Author: nicosuave