fix(node-resolve): fix error when custom resolve option missing

[tjenkinson]

Rollup Plugin Name: node-resolve

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers: fixes #1023

Description

I think older rollup versions will not provide custom, so we should handle it being undefined, as we did before #1016

There is not test for this, but to test it we either need to fake a call or somehow being in an older rollup version for the test.

[shellscape]

Unless there's a 2.x version of rollup which also lacks that, we don't have to account for this scenario. see: #1023 (comment)

[tjenkinson]

Ah yep good spot. Not sure when custom was added?

[tjenkinson]

custom was added in rollup 2.29.0 and our peer dependency is ^2.42.0, so closing this