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, 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.

/cc @dbaker