Engine resources aren't available if only custom filters engine is enabled

[seia-soto]

If the only enabled engine is "custom filters", the length of array will be 1 and no resources will be copied to the final engine.

ghostery-extension/src/utils/engines.js

Lines 436 to 444 in 25e762d

	if (engines.length > 1) {
	engine = FiltersEngine.merge(engines, {
	skipResources: true,
	overrideConfig: { enableCompression: false },
	});
	engine.resources = Resources.copy(engines[0].resources);
	} else {
	engine = engines[0];
	}

The behaviour can be tested on https://seia-soto.github.io/checkadb/ on Firefox with every recent Ghostery extension versions including #2829. The instructions are as below.

1. Disable all features: ad-blocking, anti-tracking, never-consent, and all regional filters
2. Apply /gen/rediradsbygoogle.js^$redirect=googlesyndication_adsbygoogle.js to the custom filters
3. Open the testing page and check the entry: /gen/rediradsbygoogle.js^$redirect=googlesyndication_adsbygoogle.js under networking section.

The expected result should be pass here.