Fix #24: fix handling of null field conditions

[Technici4n]

Hello, here is a proposed fix for #19 and #24.

Here are the changes:

• Undo the changes in 54c49e6. I believe that commit changes the original NOT NOT _exists_:field to NOT _exists_:field, which is incorrect as it flips the condition. My bad for not noticing when I saw the original commit!
• Fix #19 by overriding convert_condition_not to generate _exists_:field instead for that specific double NOT case.
• The parentheses around that _exists_:field are not needed when it appears in an expression, so override compare_precedence to skip parentheses if it is given a field null condition.
• Also override compare_precedence to add parentheses around other cases of NOT _exists_:field to prevent weird issues with the precedence of an unenclosed NOT in Lucene when used in AND/OR expressions.[^1]

[^1]: The LuceneBackend already generates (NOT fieldA:valueA) AND (NOT fieldB:valueB), so it makes sense to enclose (NOT _exists_:field).

[thomaspatzke]

Thanks for your contribution!