Make ReturnMissingNullable a warning

PiperOrigin-RevId: 502710013
Change-Id: I427e2b2055344e03549dea1d17f83dc589576f0b

toolchains/default_java_toolchain.bzl

diff --git a/toolchains/default_java_toolchain.bzl b/toolchains/default_java_toolchain.bzl
index 9c2b1fc..4e21aa4 100644
--- a/toolchains/default_java_toolchain.bzl
+++ b/toolchains/default_java_toolchain.bzl
@@ -58,6 +58,7 @@
     "-Xep:IgnoredPureGetter:OFF",
     "-Xep:EmptyTopLevelDeclaration:OFF",
     "-Xep:LenientFormatStringValidation:OFF",
+    "-Xep:ReturnMissingNullable:OFF",
 ]
 
 # Default java_toolchain parameters