fix(flet build): honor empty platform values for remaining build settings - #7040

Failed to save: Unable to reserve cache with key setup-uv-2-x86_64-unknown-linux-gnu-ubuntu-24.04-3.12.3-pruned-03888176764e5f13338766bd0306acd2555447b5151be86f5033762f02c579e7, another job may be creating this cache.