Improve readme plugin name header check

[ernilambar]

Few changes are there regarding plugin name header in this update https://meta.trac.wordpress.org/changeset/13162

• Whenever invalid_plugin_name_header warning is triggered, name property is false.
• This warning is triggered in two cases: when there is no valid plugin name header (and there are other content to be parsed in the readme file) AND when header is === Plugin Name ===. So I have updated the message considering both cases.
• When there is no warning invalid_plugin_name_header but $parser->name is still empty() then it means readme file is completely empty.

[ernilambar]

cc: @swissspidy @mukeshpanchal27