Improve warning for wildcard matching only null under the explicit nulls flag (scala#21577) #32969

Job	Run time
test_non_bootstrapped	0s
test	54m 32s
test_scala2_library_tasty	0s
test_windows_fast	32m 43s
MiMa	12m 25s
test_windows_full	0s
community_build_a	52m 15s
community_build_b	1h 24m 53s
community_build_c	1h 0m 8s
test_sbt	17m 56s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	7m 46s
test-msi-package	0s
build-chocolatey-package / build	24s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 23m 2s

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