dataForColumn: returns NULL when the column contains zero-length NSData

[jmatx]

If I store [NSData data] (a zero-length NSData value) into a column, then
dataForColumn: returns NULL. It should return a zero-length NSData.

[robertmryan]

Good point. It really should distinguish between NULL values and zero length BLOB objects.

By way of background, the sqlite3_column_blob documentation tells us:

The return value from sqlite3_column_blob() for a zero-length BLOB is a NULL pointer.

And the dataForColumnIndex method (which is called by the dataForColumn method) detects this situation. This method, however, returns nil in this case and there's a compelling argument for it returning [NSData data] instead.

The tactical issue is that FMDB has a large install-base, and we probably don't want to just change this behavior for fear of breaking existing code. There may be people out there who have written code looking for nil, so we should be circumspect about changing this behavior.

If this behavior is changed, I'd suggest deferring this until a major release of the library which probably will have a number of non-backward compatible changes. This should not be a change that is introduced in a minor update of the library.

[ccgus]

I'll consider this for 3.0