Log when the context does not exist instead of throwing errors

[strmer15]

We were switching from react-helmet to react-helmet-async, but our production application threw errors at runtime (effectively shutting our website down) because of a misplaced <Helmet> tag outside of the <HelmetProvider>. This caused react-helmet-async to throw the error Cannot read properties of undefined (reading 'add').

While that's an obvious problem that can be addressed separately, it would help users who are switching to check for a missing context beforehand and not attempt to reference objects that are possibly undefined. This change checks to see if helmetInstances is truthy and, if not, logs a trace to the console and returns immediately. This way, users can be aware of their code not behaving correctly while still maintaining a running website.

[strmer15]

@staylor Could you review this and let me know if it can make into the next release? Thanks!