fix(nix): omit FREESMLAUNCHER_JAVA_PATHS when jdks is empty

[wongcallum]

When I tried overriding the freesmlauncher package to set jdks = [], I got:

error: Cannot build '/nix/store/ysrqcwzv8zg9sis5linrhpvnc9hyl79d-freesmlauncher-616e092.drv'.
       Reason: builder failed with exit code 1.
       > <stdin>:61:6: error: #error makeCWrapper: Unknown argument PATH
       > <stdin>:62:6: error: #error makeCWrapper: Unknown argument :
       > <stdin>:63:6: error: #error makeCWrapper: Unknown argument /nix/store/...

This can be fixed by only setting FREESMLAUNCHER_JAVA_PATHS if the list is not empty.

[s0me1newithhand7s]

i think it's better to make assertion to not let jdks be empty at all

[wongcallum]

But I want to use my own Java :(