Avoid PHP type deprecation on function call

[herndlm]

get's rid of

1) phpmock\integration\MockDelegateFunctionBuilderTest::testBuild
function_exists(): Passing null to parameter #1 ($function) of type string is deprecated

/Users/herndlm/Development/source/git-forks/php-mock-integration/tests/MockDelegateFunctionBuilderTest.php:23

2) phpmock\integration\MockDelegateFunctionTest::testDelegateReturnsMockResult
function_exists(): Passing null to parameter #1 ($function) of type string is deprecated

/Users/herndlm/Development/source/git-forks/php-mock-integration/tests/MockDelegateFunctionTest.php:38

3) phpmock\integration\MockDelegateFunctionTest::testDelegateForwardsArguments
function_exists(): Passing null to parameter #1 ($function) of type string is deprecated

/Users/herndlm/Development/source/git-forks/php-mock-integration/tests/MockDelegateFunctionTest.php:56

which comes from calling https://github.com/php-mock/php-mock/blob/2.4.0/classes/generator/ParameterBuilder.php#L32 with null. According to that PHPDoc this is invalid already anyways and static analysis would catch that.

[michalbundyra]

Thanks, @herndlm !