feat(static-website): allow empty params

[fargito]

Follow-up of #213 which was unfortunately closed

What I need to do is parametrize some of my config:

  constructs: {
    app: {
      type: 'static-website',
      path: '../build',
      errorPage: '404.html',
      domain: '${param:domain}',
      redirectToMainDomain: '${param:redirectToMainDomain}',
      certificate: '${param:certificate}',
    },
  },

However if I try to setup my params with null values, they get cleaned out from stage params.

  params: {
    dev: {
      domain: [],
      redirectToMainDomain: false,
      certificate: null, // issue here
    },
    production: {
      domain: ['www.mywebsite.com', 'mywebsite.com'],
      redirectToMainDomain: true,
      certificate: 'my-certificate-arn',
    },
  },

If I do this, I get the following error:

And if I try to put the params like:

  params: {
    dev: {
      domain: [],
      redirectToMainDomain: false,
      certificate: '' // with an empty string
    },
    production: {
      domain: ['www.mywebsite.com', 'mywebsite.com'],
      redirectToMainDomain: true,
      certificate: 'my-certificate-arn',
    },
  },

I get the following error:

This PR adds the possibility to pass empty arguments for certificate and domain configurations.

[mnapoli]

OK for me 👍