is_array check on null in updateCache method in bedrock-autoloader.php not compatible with php 7.4

[sanderdv]

• I've read the guidelines for Contributing to Roots Projects
• This request isn't a duplicate of an existing issue
• I've read the docs and followed them (if applicable)
• This is not a personal support request that should be posted on the Roots Discourse community

Description

This issue happens when using php 7.4 (as roots/trellis 1.4.0 does). In /web/app/mu-plugins/bedrock-autoloader.php on line 176 in the updateCache method of the Autoloader class. There is a check if $this->cache['plugins'] is an array by using is_array. However, $this->cache['plugins'] can be null at that point. As of php 7.4 is_array will no longer return null when the given variable is null, but throws a notice for invalid array access.

I was able to fix it by first checking if $this->cache is an array, like this:
$rebuild = !(is_array($this->cache) && is_array($this->cache['plugins']));

Steps to reproduce

1. Use Bedrock in combination with php 7.4 (roots/trellis 1.4.0 for example).
2. Make sure you can see php notices.
3. Add a mu-plugin in the mu-plugin folder.
4. See the notices appear when opening the site.

Expected behavior:
No php exceptions or notices when using bedrock and php 7.4

Actual behavior:
A php notice when using bedrock and php 7.4

Versions

Bedrock 1.13.2
Trellis 1.4.0
OSX 10.14.6

[Log1x]

Thanks!