(maint) Add specs for parser resource parameters

Classes Puppet::Parser::AST::ResourceParam and Puppet::Parser::Resource::Param
(#23115) Include manifest context in Puppet::DevError

When a 'devfail' or Puppet::DevError is thrown, include the user's manifest
file/line context as it can help in finding the problem.
(#23115) Convert nil resource parameter values to undef

If a resource parameter evaluates to nil (e.g. a return value from a parser
function), then convert the value to undef.  This prevents an error in
Puppet::Parser::Resource::Param which requires a non-nil value on
instantiation.
Merge pull request #2049 from domcleal/tickets/master/23115-param-nil

(#23115) Convert nil resource parameter values to undef.