quoting values magic

Assume I have an UPDATE statement that looks like

UPDATE foo
SET
pattern = '$pattern',
shape = '$shape',
length = $length,
comment = '$comment'
WHERE foo_id = $foo_id

and length is defined as NUMERIC. Is there any kind of magic that
would allow me to use the SQL above as is even if $length is not
defined? In other words, I'd like to avoid having to modify the SQL
to include or not include "length = $length" based on whether or not
$length is defined as it's acceptable for it to be NULL in foo.

I can't say "length = '$length'" as '' is not valid input for NUMERIC.

Hope that makes sense?

Thanks.

--
Brandon