Inputs: do not run nvm if not needed

[jsmrcaga]

Remove NVM execution and installation if it is not required:

• if user did not set a specific node version
• if repo does not possess a .nvmrc file

Linked to issue: https://github.com/jsmrcaga/action-netlify-deploy/issues/27

[jsmrcaga]

@tpluscode could you review this PR and check if it still matches your use-cases ?

[tpluscode]

Yes, that works for me!