FilePropertyKey should not modify strings that are going to be persisted

This is not expected by client code. For example, TemplateDataLanguagePusher pushes Language.ANY which is serialized to empty string "". `FilePropertyKey` transformed empty string into `null` and that `null` never deserializes into Language.ANY.

Now FilePropertyKey does not modify passed strings and stores them "as is"

GitOrigin-RevId: eb388afbabcb2fde9c0fb979d4053e5815796e7c

platform/analysis-impl/src/com/intellij/psi/FilePropertyKeyImpl.kt

@@ -160,9 +160,7 @@ internal class FilePropertyStringKey<T>(name: String, persistentAttribute: FileA
   override fun readValue(stream: AttributeInputStream): String? = stream.readEnumeratedString()
 
   @Throws(IOException::class)
-  override fun writeValue(stream: AttributeOutputStream, newValue: String?) {
-    StringUtil.nullize(newValue).let { stream.writeEnumeratedString(it) }
-  }
+  override fun writeValue(stream: AttributeOutputStream, newValue: String?) = stream.writeEnumeratedString(newValue)
 }
 
 internal class FilePropertyEnumKey<T : Enum<T>>(name: String,

platform/platform-tests/testSrc/com/intellij/psi/FilePropertyKeyImplTest.java

@@ -18,6 +18,7 @@ import org.junit.runners.Parameterized.Parameter;
 
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.List;
 
 @RunWith(Parameterized.class)
 public class FilePropertyKeyImplTest<T> extends LightPlatformTestCase {
@@ -132,6 +133,26 @@ public class FilePropertyKeyImplTest<T> extends LightPlatformTestCase {
     TestCase.assertNull("We still can read null", key.getPersistentValue(file));
   }
 
+  @Test
+  public void testStringNotModifiedWhenPersisted(){
+    Assume.assumeTrue(key == STRING_KEY);
+    VirtualFile file = createVirtualFile("Foo.java", "");
+
+    List<String> values = Arrays.asList("value", "s p a c e s", "," ,"", " ", "\t", "null", null);
+    for (String value : values) {
+      STRING_KEY.setPersistentValue(file, value);
+      assertEquals("Should read exactly the same string (read from memory)", value, STRING_KEY.getPersistentValue(file));
+
+      memKey.set(file, null); // clear memory data
+      assertFalse("Should not update to the same string: '" + value + "'", STRING_KEY.setPersistentValue(file, value));
+      assertEquals("Should read exactly the same string (read from memory)", value, STRING_KEY.getPersistentValue(file));
+
+      if (!FilePropertyKeyImpl.getREAD_PERSISTENT_VALUE()) continue;
+      memKey.set(file, null); // clear memory data
+      assertEquals("Should read exactly the same string (read from file)", value, STRING_KEY.getPersistentValue(file));
+    }
+  }
+
   @Parameterized.Parameters(name = "{0}")
   public static Collection samples() {
     return Arrays.asList(new Object[][]{