Do not return empty strings in hash if option in datastore is nil

[sjanusz-r7]

This is in draft for now as I explore other options to fix this issue in Metasploit Pro.

This PR fixes an issue in Metasploit Pro.

With this PR, the datastore no longer coalesces nil values into empty strings. For example:

datastore['TEST_VAL'] = nil
datastore['USERNAME'] = 'rapid7'

datastore_hash = datastore.to_h
datastore_hash == { 'TEST_VAL' => '', 'USERNAME' => 'rapid7' }

Now, nil values are ignored:

datastore['TEST_VAL'] = nil
datastore['USERNAME'] = 'rapid7'

datastore_hash = datastore.to_h
datastore_hash == { 'USERNAME' => 'rapid7' } # TEST_VAL is not present

The semantics of assigning an empty string are unchanged. E.g.:

datastore['EMPTY'] = ''

datastore_hash = datastore.to_h
datastore_hash == { 'EMPTY' => '' }

This results in the expected results of:

datastore_hash['TEST_VAL'] == nil

Example from the RSpec tests:

[4] > subject
=> #<Msf::ModuleDataStoreWithFallbacks:0x000000011a458538
 @_module=#<InstanceDouble(Msf::Exploit) (anonymous)>,
 @aliases={},
 @defaults={},
 @options={},
 @user_defined={"foo"=>"foo_value", "bar"=>"bar_value", "empty"=>"", "example_nil"=>nil}>
[5] > subject.to_h
=> {"foo"=>"foo_value", "bar"=>"bar_value", "empty"=>""}

Verification

• Start msfconsole
• Ensure you can run a module with the correct options
• Point Pro at this Framework commit
• Ensure you can replay a session using psexec in Pro
• Run framework tests using bundle exec rspec spec/lib/msf/core/data_store_with_fallbacks_spec.rb

[adfoster-r7]

I think you're accidentally trying to fix the side effect of this decision:

metasploit-framework/lib/msf/core/data_store_with_fallbacks.rb

Lines 263 to 272 in deef85d

	# Override Hash's to_h method so we can include the original case of each key
	# (failing to do this breaks a number of places in framework and pro that use
	# serialized datastores)
	def to_h
	datastore_hash = {}
	self.keys.each do |k|
	datastore_hash[k.to_s] = self[k].to_s
	end
	datastore_hash
	end

This PR might not be the correct fix for your original problem, maybe worth a double check 👀