Fix reactOnRailsPageUnloaded when there is no component on the page

[justin808]

From #1498.

In reactOnRailsPageLoaded, if we finds no react component, the function return and context.roots is not initialized. This cause findContext().roots in reactOnRailsPageUnloaded to be undefined, and for .. of throws an exception.

This should mostly affect app using both hotwire and react_on_rails, as the recommended way to load js is to do the same file everywhere, otherwise the issue can be avoided by just not loading react_on_rails

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This change is 

[justin808]

Merged #1498.