(#23369) Allow caller to override YAML.load_file returning false

According to https://github.com/tenderlove/psych/issues/149, Syck and
Psych return false when ::YAML.load_file tries to parse an empty file.

Since Puppet::Indirector::Yaml#find calls Puppet::Util::Yaml.load_file
directly, we can't change the semantics of what it returns.

Instead we allow the caller to explicitly specify what the default value
should be.