[JVM IR] Maintain KT-36625 bug compatibility between non-IR and IR

backends by removing IMPLICIT_NOTNULL casts from IrStringConcatenation
arguments.

Also fixed an issue where IrStringConcatenation can be lowered into
a null String instead of a literal "null" String if the lone argument
was a platform type String or String with enhanced nullability and the
value was null (e.g., "${FromJava.nullPlatformString()}").

compiler/tests/org/jetbrains/kotlin/codegen/BlackBoxCodegenTestGenerated.java

@@ -27490,6 +27490,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             runTest("compiler/testData/codegen/box/strings/stringBuilderAppend.kt");
         }
 
+        @TestMetadata("stringFromJavaPlus.kt")
+        public void testStringFromJavaPlus() throws Exception {
+            runTest("compiler/testData/codegen/box/strings/stringFromJavaPlus.kt");
+        }
+
         @TestMetadata("stringPlusOnlyWorksOnString.kt")
         public void testStringPlusOnlyWorksOnString() throws Exception {
             runTest("compiler/testData/codegen/box/strings/stringPlusOnlyWorksOnString.kt");