nvidia: suppress newline-only RM log output

Fixes #1098. out_string() forwards strings from RM directly to printk(). If RM emits a separator consisting only of a newline, this can create a blank kernel log record during boot and appear in journalctl -p3 as a false error. Ignore NULL, empty, and newline-only strings before calling printk(). Real log messages are still printed unchanged. Although the visible symptom is a log artifact, this changes executable logging behavior by suppressing separator-only output before printk(). Validation: - git diff --check Not performed: - Local module build - Runtime boot-log test