PHPLIB-1384: Use default value in Query::exists()

[alcaeus]

PHPLIB-1384

I decided to add another test to explicitly test using exists with a different value.

[GromNaN]

I'm fine with keeping the explicit Query::exists(true) in some tests, but we should have static analysis warnings for setting the default argument value.

mongo-php-builder/tests/Builder/Query/AndOperatorTest.php

Line 26 in 214baf1

price: Query::exists(true),

mongo-php-builder/tests/Builder/Stage/FacetStageTest.php

Line 43 in 214baf1

price: Query::exists(true),

mongo-php-builder/tests/Builder/Stage/CurrentOpStageTest.php

Line 26 in 214baf1

transaction: Query::exists(true),