Fix ability to add custom frame colors

[garrettjstevens]

If you add a custom frame theme to your config.json like this:

{
  "configuration": {
    "theme": {
      "palette": {
        "frames": [
          null,
          { "main": "#FF8080" },
          { "main": "#80FF80" },
          { "main": "#8080FF" },
          { "main": "#8080FF" },
          { "main": "#80FF80" },
          { "main": "#FF8080" }
        ]
      }
    }
  }
}

you get an error:

TypeError: Cannot convert undefined or null to object
TypeError: Cannot convert undefined or null to object
    at Function.keys (<anonymous>)
/path/to/jbrowse-components/products/jbrowse-web/src/util.ts:135:1

This is because there's a null in the config, which I don't think we have anywhere else, and the addRelativeUris function was missing a null check. This PR adds the null check.

[cmdcolin]

excellent. could be worth adding to some docs example too