Fix: goheader linter can throw nil pointer exception in case of a source file has not issues

[denis-tingaikin]

Description

During local testing latest golangci, we've found that go-header linter can throw nil pointer exception in case of a source file is valid.

What is this patch doing?

• Fix issue.
• Add test to cover.
• Allow using empty new lines in testdata/.*\.go files.

Steps to reproduce the issue

1. Use next go-header config

linters-settings:
  goheader:
    template: "test"

2. Run golangci linter on file with valid header, for example

//test

//Package mypkg ...
package mypkg

Actual: nil pointer exception
Expected: Template has matched no issues should be found

[denis-tingaikin]

Here is failed one check that is fixed in #1207

[denis-tingaikin]

@sayboras, @iwankgb, I interested to release a new version with this patch dfa0013, Do you have any idea what I need to do? :)

[iwankgb]

This is bugfix release so:

• Push v1.28.1 tag.
• follow steps outlined in Automate update for github-action-config.json #1208

[denis-tingaikin]

@iwankgb thanks! Done! Also I got a question where we should put generated docs/.*mdx files?

[iwankgb]

@denis-tingajkin I don't think I understand what you mean. mdx files gets published automatically to the docs site.

[denis-tingaikin]

@iwankgb I meant generated files after make expand_website_templates. Should I also create PR to the website repository?