Switch the core library to be non-null by default

See comments in c8c128d335 for further
details. This requires /relatively/ few changes - mostly cases we were
missing @Nullable annotations.

projects/core/build.gradle.kts

@@ -5,6 +5,7 @@ plugins {
     id("cc-tweaked.kotlin-convention")
     id("cc-tweaked.java-convention")
     id("cc-tweaked.publishing")
+    id("cc-tweaked.errorprone")
     id("cc-tweaked")
 }