• Yes, I reviewed the contribution guidelines.
• Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Describe the current, buggy behavior
When installing a WordPress core language using wp core language install [...] with WP CLI nightly release (2.5.0-alpha-1a19028), a PHP notice is emitted:

In FileCache.php line 325:
                                                       
  Trying to access array offset on value of type null

Describe how other contributors can replicate this bug

• PHP 7.4.16
• WP CLI 2.5.0-x
  Install a WordPress core language that isn't already installed.

Let us know what environment you are running this on

OS:     Linux 5.8.0-50-generic wp-cli/language-command#56-Ubuntu SMP Mon Apr 12 17:18:36 UTC 2021 x86_64
Shell:  /bin/bash
PHP binary:     /usr/bin/php7.4
PHP version:    7.4.16
php.ini used:   /etc/php/7.4/cli/php.ini
MySQL binary:   /usr/bin/mysql
MySQL version:  mysql  Ver 15.1 Distrib 10.5.8-MariaDB, for debian-linux-gnu (x86_64) using readline 5.2
SQL modes:
WP-CLI root dir:        phar://wp-cli.phar/vendor/wp-cli/wp-cli
WP-CLI vendor dir:      phar://wp-cli.phar/vendor
WP_CLI phar path:       /srv/www/example.com/releases/20210422164903
WP-CLI packages dir:
WP-CLI global config:
WP-CLI project config:  /srv/www/example.com/releases/20210422164903/wp-cli.yml
WP-CLI version: 2.5.0-alpha-1a19028

** Related **
Apparently the notice above isn't always logged (there is an unrelated issue that causes PHP infos/warnings to be treated as fatal errors that break execution). However, when the notice is not logged, the following warning is logged instead:

Warning: Failed to create directory '/home/web/.wp-cli/cache/': mkdir(): Permission denied.

Ensuring the right permissions (/ownership) of the directory fixed the notice. However, this would be only a workaround for this issue because the variable is used as an array in situations where the variable is actually null, like in this case where mkdir in WP CLI failed.