fix: fix broken workflows (#5634)
[TML-1495](https://linear.app/prisma-company/issue/TML-1495/fix-broken-workflows)
Passing undefined parameters through workflows causes them to be
defaulted to '', which fails parameter validation in downstream
workflows. I changed them to use `||` for the defaults instead and
removed the defaults from the workflow definitions.
Fixes these:
https://github.com/prisma/prisma-engines/actions/runs/18374070104
https://github.com/prisma/prisma-engines/actions/runs/18352196444
The fix can be observed by the fact that for example `pg (wasm)` is run
within this CI, but it wasn't in the last main CI
(https://github.com/prisma/prisma-engines/actions/runs/18374070202/job/52343845457).
