Make decrypt-value and encrypt-value mojos standalone runnable

[jonas-haeusler]

https://github.com/ulisesbocchio/jasypt-spring-boot/pull/284 introduced standalone runnability for the encrypt-value mojo. https://github.com/ulisesbocchio/jasypt-spring-boot/pull/220 introduced a regression that required the ./src/main/resources/ file structure to be present to be able to run a mojo, effectively breaking the standalone runnability (and also the runnability from e.g. the root of a multi module project).

This PR adds the same standalone runnability for the decrypt-value mojo as present for the encrypt-value mojo and fixes the regression by making the config path optional.