fix: cover the old default value 1.6 (same as 1.6.4) with empty migration

[samrose]

previously, the build scripts from pg_cron extension had output migrations patterned like 1.5-1.6 etc

recently we rewrote the packages to support multiple versions, and in the process did not have coverage for this form of extension version.

This adds an empty migration from 1.6 - 1.6.4 to cover where people have the previous version signature.

[samrose]

working on testing this now