fix(prebuilt/alloydb-omni): require password env var explicitly (#3398)

## Description

The `alloydb-omni` prebuilt config declared `password:
${ALLOYDB_OMNI_PASSWORD:}` (empty default), but the underlying
`postgres` source marks `Password` as `validate:"required"`. An unset
`ALLOYDB_OMNI_PASSWORD` therefore resolves to `""` → `null` and fails
config validation anyway — the empty default was misleading.

This drops the empty default and declares the var bare-required
(`${ALLOYDB_OMNI_PASSWORD}`), matching the adjacent `user` field.
Behavior is unchanged for the served path (an unset password already
failed); it just makes the requirement honest and lets offline tooling
substitute the placeholder.

Stacked on #3388.

Author: twishabansal

internal/prebuiltconfigs/tools/alloydb-omni.yaml

@@ -19,7 +19,7 @@ host: ${ALLOYDB_OMNI_HOST:localhost}
 port: ${ALLOYDB_OMNI_PORT:5432}
 database: ${ALLOYDB_OMNI_DATABASE}
 user: ${ALLOYDB_OMNI_USER}
-password: ${ALLOYDB_OMNI_PASSWORD:}
+password: ${ALLOYDB_OMNI_PASSWORD}
 queryParams: ${ALLOYDB_OMNI_QUERY_PARAMS:}
 ---
 kind: tool