Expected:
INDEX ON ... FOR ... CANDIDATE don't fire a null error when a FOR filter it filters the null keys.

Observed:
a filtered index uses two different ways:
- way 1: it applies FOR and only if it is true it inserts the key in the file.
this is used in the commands
INDEX ON
REINDEX
( this is ok for a performance gain )
- way 2: insert/update the key and after mark it with the flag gotten from
the FOR filter;
this has as a result that changing the order
of the lines different results are gotten.
( this is inevitable however,
but the filter can be calculated before and to mark the keys as "out of set")

Repro:
Write the same program into two ways:

* Way #1, this work:

clear
CREATE CURSOR test (f1 I NULL)
TRY
	INSERT INTO test VALUES (NULL)
	INDEX ON F1 FOR F1>0 TAG T1 CANDIDATE && this filters the Nulls
	SET ORDER TO 0
CATCH
	? MESSAGE()
ENDTRY
BROWSE

* Way #2, this doesn't work:

clear
CREATE CURSOR test (f1 I NULL)
TRY
	INDEX ON F1 FOR F1>0 TAG T1 CANDIDATE
	INSERT INTO test VALUES (NULL)
	SET ORDER TO 0
CATCH
	? MESSAGE()
ENDTRY
BROWSE

solutions:
- CANDIDATE reject nullable keys ( F1 is nullable ) NO GOOD FOR ME

- CANDIDATE it accepts filter null keys GOOD FOR ME

Enhancement:
- CANDIDATE it accept one null not filtered VERY GOOD for work with a editing new row