Improve warning for wildcard matching only null under the explicit nu… #32970

Job	Run time
test_non_bootstrapped	25m 27s
test	54m 58s
test_windows_fast	0s
test_scala2_library_tasty	0s
test_windows_full	43m 24s
MiMa	13m 2s
community_build_a	50m 50s
community_build_b	1h 27m 19s
community_build_c	1h 2m 20s
test_sbt	20m 59s
test_java8	0s
build-msi-package	0s
build-sdk-package / build	7m 51s
test-msi-package	0s
build-chocolatey-package / build	22s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	6h 6m 32s

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