Right now, the multiversion passthrough suites are testing 4.4/5.1 mixed version clusters. However, null_query_semantics.js tests behaviors added under SERVER-21929 that are present only in the master branch and in 5.0. Therefore, this test should not run in the multiversion passthrough suites until those suites start testing 5.0/5.1 mixed version clusters.

This can be achieved by tagging the test as requires_fcv_50. Once other work is done to bump the FCV constants and start testing 5.0/5.1 multiversion clusters (see SERVER-56366), then tests with requires_fcv_50 will start running in the multiversion passthroughs again. At some later point, we can then delete the obselete requires_fcv_50 tag.