meson: remove "empty array"/"array of an empty string" confusion

Until Meson 0.47, setting `-D arrayoption=` was not the same as setting
`-D arrayoption=[]`; the latter cleared the array, while the former
filled it with an empty string option.

Since Meson 0.47 [1], the former maps to the latter, so empty items can
only be set by explicitly giving an array containing an empty string,
ie. `-D arrayoption="['']"`; however note that this is *not* what we
want in any of the current Mesa code anyway.

This makes the code handling array options a bit more complicated, and
a lot more error-prone, so let's get rid of the confusion by removing
the empty-string option.

[1] https://github.com/mesonbuild/meson/commit/f3a8f9c34d95d862fb4d12869a7b31cea592561f



Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <mesa/mesa!386>