Treat host as possibly undefined for base64encode/base64decode

[kitsonk]

Fixes: #24638

Does not assume that ts.sys (a.k.a. host) is always defined, because when running on non-node/ChakraCore runtimes (e.g. deno), .sys does not get defined currently.

Here's a checklist you might find useful.

• There is an associated issue that is labeled
  'Bug' or 'help wanted' or is in the Community milestone
• Code is up-to-date with the master branch
• You've successfully run jake runtests gulp runtests locally
• You've signed the CLA
• There are new or updated unit tests validating the change¹

Fixes: #24638
Refs: ry/deno#117

[1]: I couldn't find any unit tests that were appropriate to update or add for this area of code. Would be glad to if someone could point out a related unit test.