Add a test for when there is a missing value in the data

tests/testthat/test-Zalpha.R

tests/testthat/test-Zalpha_BetaCDF.R

tests/testthat/test-Zalpha_Zscore.R

tests/testthat/test-Zalpha_all.R

tests/testthat/test-Zalpha_log_rsq_over_expected.R

tests/testthat/test-Zalpha_rsq_over_expected.R

tests/testthat/test-Zbeta.R

tests/testthat/test-Zbeta_BetaCDF.R

tests/testthat/test-Zbeta_Zscore.R

tests/testthat/test-Zbeta_log_rsq_over_expected.R

tests/testthat/test-Zbeta_rsq_over_expected.R