(#6107) Fix an error when auditing a file with empty content

The manifest:

file { "/tmp/foo" :

ensure => present,
audit => content,

}

produced the error:

err: /Stage[main]//File[/tmp/foo]/ensure: change from absent to present
failed: Could not retrieve content for from filebucket: private method `sub' called for nil:NilClass at
/Users/matthewrobinson/work/puppet/test.pp:4

This was due to logic in content assuming that if you didn't specify
content while you were auditing it you must have specified a source.

The code paths in the file type badly need a cleanup so that these sorts
of errors aren't so difficult to track down and things are easier to
test.

Paired-with: Daniel Pittman