feat(webapp,cli): support empty-string environment variable values

Adds support for environment variables with empty-string values, rolling
out gradually. Once enabled for your organization, an empty value stays
stored and available to tasks, including through environment-variable
imports and Vercel sync. Deleting a variable remains a separate action.
Also fixes local dev environment precedence so freshly resolved project
values reach task processes correctly.

Mono-RevId: 25dea78a2c72a984a9bd9ac432c11026f220576c