The annotation processor detects `@ConfigurationProperties` bean or
method definition and merges manual meta-data. The former step will fail
with a NPE if the annotation is not present on the classpath. This could
happen if the annotation processor is added to a module that is not
actually using Spring Boot.

We now have a defensive check that skips that steps but still attempts to
merge manual meta-data if present.

Closes gh-3720