Enhance filter handling for OR expressions and simplify null cases

[mholthausen]

This fixes cross-filtering on <NULL> values and closes #56 .

When selecting <NULL> / N/A in superset cross-filters, superset can generate SQL like:

sperrart IS NULL OR sperrart IN (NULL) AND (1 != 1)

Those patterns are valid SQL but not directly valid as WFS property filters.

Therefore, the dialect previously failed in two ways:

1. OR expressions were not supported at all
2. SQLAlchemy expressions (IN (NULL), 1 != 1) were tranformed literally to FES, which led to invalid WFS filters (e.g. ValueReference=1) and server errors

With the changes in this PR OR support in filter conversion and a filter pre-processing for special cases IN (NULL) and 1 != 1 are added here.

Now expressions like sperrart IS NULL OR sperrart IN (NULL) AND (1 != 1) are reduced to a valid FES filter (sperrart IS NULL) before tranformation.