Fix console error in examples pages due to no theme switcher

[julien-deramond]

Description

This PR checks in color-mode.js that the themeSwitcher var is not null before continuing the algorithm.

Motivation & Context

This avoids an error message in the console when one goes to any examples in our doc (e.g. https://twbs-bootstrap.netlify.app/docs/5.3/examples/heroes/) either in light or dark mode:

Uncaught TypeError: btnToActive is null
    showActiveTheme https://twbs-bootstrap.netlify.app/docs/5.3/assets/js/color-modes.js:35
    <anonymous> https://twbs-bootstrap.netlify.app/docs/5.3/assets/js/color-modes.js:60
color-modes.js:35:28

This is because our examples react to the theme being light or dark via color-modes.js, but they don't have any dropdown switcher on the page (like in the header for other pages).

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38074--twbs-bootstrap.netlify.app/docs/5.3/examples/heroes/ (or any other example)