Skip placeholder creation when source-config has no snapshots

Skip placeholder creation when source-config has no snapshots

scripts/snapper-sync derives the highest snapshot number for each source-config from snbk list and then creates a placeholder named after that number under <subvolume>/.snapshots/. When no snapshots match, jq prints the literal string "null" for the existing max_by query, which previously led to a .snapshots/null directory.

Keep the original jq query unchanged and skip placeholder creation when max_num is "null". Also quote "$outputs" on the line that feeds the JSON list into jq, preserving the existing shellcheck SC2086 fix.

Fixes #1131