Enable explicit nulls for most community projects

Author: HarrisL2

community-build/src/scala/dotty/communitybuild/projects.scala

@@ -138,6 +138,7 @@ object SbtCommunityProject:
   def scalacOptions = List(
     "-Xcheck-macros",
     "-Wsafe-init",
+    "-Yexplicit-nulls",
   )
 
 object projects:
@@ -277,7 +278,8 @@ object projects:
     // Problem parsing scalatest.dotty/target/scala-3.0.0-M2/src_managed/main/org/scalatest/concurrent/ConductorFixture.scala:[602..624..3843], documentation may not be generated.
     // dotty.tools.dotc.core.MissingType:
     dependencies = List(scalaXml),
-    testOnlyDependencies = () => List(scalatestplusJunit, scalatestplusTestNG)
+    testOnlyDependencies = () => List(scalatestplusJunit, scalatestplusTestNG),
+    scalacOptions = SbtCommunityProject.scalacOptions.filter(_ != "-Yexplicit-nulls")
   )
 
   lazy val scalatestplusScalacheck = SbtCommunityProject(
@@ -306,7 +308,8 @@ object projects:
     project       = "scala-xml",
     sbtTestCommand = "xml/test",
     sbtPublishCommand = "xml/publishLocal",
-    sbtDocCommand = "xml/doc"
+    sbtDocCommand = "xml/doc",
+    scalacOptions = SbtCommunityProject.scalacOptions.filter(_ != "-Yexplicit-nulls"),
   )
 
   lazy val scalap = SbtCommunityProject(