Guard against undefined reference in webpack plugin

[jeffposnick]

R: @philipwalton

Fixes #2194

The code in question should be safe to skip if the condition is false.

[workbox-pr-bot]

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File	Before	After	Change	GZipped
packages/workbox-background-sync/build/workbox-background-sync.prod.js	3.87 KB	3.87 KB	0%	1.60 KB
packages/workbox-broadcast-update/build/workbox-broadcast-update.prod.js	1.02 KB	1.02 KB	0%	576 B
packages/workbox-build/build/_types.js	41 B	41 B	0%	61 B
packages/workbox-build/build/generate-sw.js	1.99 KB	1.99 KB	0%	966 B
packages/workbox-build/build/get-manifest.js	1.60 KB	1.60 KB	0%	779 B
packages/workbox-build/build/index.js	613 B	613 B	0%	344 B
packages/workbox-build/build/inject-manifest.js	3.13 KB	3.13 KB	0%	1.36 KB
packages/workbox-cacheable-response/build/workbox-cacheable-response.prod.js	610 B	610 B	0%	356 B
packages/workbox-cli/build/app.js	4.16 KB	4.16 KB	0%	1.64 KB
packages/workbox-cli/build/bin.js	940 B	940 B	0%	502 B
packages/workbox-core/build/workbox-core.prod.js	6.42 KB	6.42 KB	0%	2.66 KB
packages/workbox-expiration/build/workbox-expiration.prod.js	2.95 KB	2.95 KB	0%	1.27 KB
packages/workbox-google-analytics/build/workbox-offline-ga.prod.js	1.97 KB	1.97 KB	0%	915 B
packages/workbox-navigation-preload/build/workbox-navigation-preload.prod.js	659 B	659 B	0%	322 B
packages/workbox-precaching/build/workbox-precaching.prod.js	4.84 KB	4.84 KB	0%	1.87 KB
packages/workbox-range-requests/build/workbox-range-requests.prod.js	1.58 KB	1.58 KB	0%	800 B
packages/workbox-routing/build/workbox-routing.prod.js	3.09 KB	3.09 KB	0%	1.35 KB
packages/workbox-strategies/build/workbox-strategies.prod.js	4.10 KB	4.10 KB	0%	1.13 KB
packages/workbox-streams/build/workbox-streams.prod.js	1.44 KB	1.44 KB	0%	697 B
packages/workbox-sw/build/workbox-sw.js	1.34 KB	1.34 KB	0%	746 B
packages/workbox-webpack-plugin/build/generate-sw.js	4.22 KB	4.22 KB	0%	1.61 KB
packages/workbox-webpack-plugin/build/index.js	349 B	349 B	0%	255 B
packages/workbox-webpack-plugin/build/inject-manifest.js	5.10 KB	5.10 KB	0%	1.77 KB
packages/workbox-window/build/workbox-window.dev.umd.js	41.51 KB	41.51 KB	0%	9.20 KB
packages/workbox-window/build/workbox-window.prod.umd.js	4.52 KB	4.52 KB	0%	1.85 KB

Workbox Aggregate Size Plugin

3.51KB gzip'ed (23% of limit)
7.93KB uncompressed