Have Lantern metrics throw when no toplevel events

[patrickhulce]

So the observed metrics throw when there are no toplevel events (like happened recently)

lighthouse/lighthouse-core/lib/traces/tracing-processor.js

Lines 178 to 182 in 0203330

	// There should *always* be at least one top level event, having 0 typically means something is
	// drastically wrong with the trace and would should just give up early and loudly.
	if (!topLevelEvents.length) {
	throw new Error('Could not find any top level events');
	}

However, the lantern metrics never directly call this method and so were silently returning lower values instead of erroring.

[patrickhulce]

fixed by #5878