The changes made in 6a2ac080 mean that getSecurity() on
ManagementServerProperties will no longer return null when Spring
Security is on the classpath. This had the unwanted side-effect of
causing the health endpoint to hide its details when Spring Security
was not on the classpath.

This commit reinstates the previous behaviour by only considering
the health endpoint to be secure if Spring Security is on the
classpath and management.security.enabled is true.

Closes gh-7345