Add rule to detect implicit nullable default values

Author: TRowbotham

.phpcs.xml

@@ -37,6 +37,7 @@
     <rule ref="SlevomatCodingStandard.Functions.StrictCall" />
     <rule ref="SlevomatCodingStandard.Functions.StaticClosure" />
     <rule ref="SlevomatCodingStandard.Operators.DisallowEqualOperators" />
+    <rule ref="SlevomatCodingStandard.TypeHints.NullableTypeForNullDefaultValue" />
 
     <!-- Cleaning - detecting dead code -->
     <rule ref="SlevomatCodingStandard.Functions.UnusedInheritedVariablePassedToClosure" />