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-api/build.gradle.kts

@@ -18,6 +18,7 @@ dependencies {
     compileOnly(project(":mc-stubs"))
     compileOnlyApi(libs.jsr305)
     compileOnlyApi(libs.checkerFramework)
+    compileOnlyApi(libs.jetbrainsAnnotations)
 
     "docApi"(project(":"))
 }