Use PHP 7.2 features in Optimization Detective

[westonruter]

In #1130 the PHP version was bumped to 7.2. This allows us to make use of new PHP language features which further harden static analysis (related to #775). Changes in this PR include:

• Use the void return type
• Use the nullable type
• Add value types for arrays
• Add improved typing to prevent having to ignore the following errors in PHPStan level 6:
  • #^(Function|Method) .+? return type has no value type specified in iterable type array#
  • #^(Function|Method) .+? has parameter .+? with no value type specified in iterable type array#
  • #^(Function|Method) .+? has no return type specified#

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[joemcgill]

Just one suggestion, but looks good. Pre-approving.

[joemcgill]

Good catch ✨