bitbake: data_smart: Fix an unusual variable reference bug

bitbake: data_smart: Fix an unusual variable reference bug

If you try:

Y = ""
Y_remove = "X"

in OE-Core, bitbake will crash with a KeyError during expansion. The reason
is that no expansion of the empty value is attempted but removal from is it
and hence no varparse data is present for it in the expand_cache.

If the value is empty, there is nothing to remove so the best fix is simply
not to check for None but check it has any value.

Also add a test for this error so it doesn't get reintroduced.

(Bitbake rev: af3ce0fc0280e6642fa35de400f75fdbabf329b1)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>