fix: convert null signal value to blank in bindText (#22915)

* fix: convert null signal value to blank in bindText

Element.setText(String) already converts null to blank. This change makes bindText to work in the same way with null signal value.

Fixes: #22898

* formatting

Author: tltv

flow-server/src/main/java/com/vaadin/flow/dom/Element.java

@@ -1286,16 +1286,16 @@ public Element setText(String textContent) {
                     "setText is not allowed while a binding for text exists.");
         }
 
-        if (textContent == null) {
-            // Browsers work this way
-            textContent = "";
-        }
         setTextContent(textContent);
 
         return this;
     }
 
     private void setTextContent(String textContent) {
+        if (textContent == null) {
+            // Browsers work this way
+            textContent = "";
+        }
         if (isTextNode()) {
             getStateProvider().setTextContent(getNode(), textContent);
         } else {

flow-server/src/test/java/com/vaadin/flow/dom/ElementBindTextTest.java

@@ -15,8 +15,11 @@
  */
 package com.vaadin.flow.dom;
 
+import java.util.LinkedList;
+
 import org.junit.After;
 import org.junit.AfterClass;
+import org.junit.Assert;
 import org.junit.Before;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -25,10 +28,10 @@
 import com.vaadin.experimental.FeatureFlags;
 import com.vaadin.flow.component.UI;
 import com.vaadin.flow.internal.CurrentInstance;
+import com.vaadin.flow.server.ErrorEvent;
 import com.vaadin.flow.server.MockVaadinServletService;
 import com.vaadin.flow.server.MockVaadinSession;
 import com.vaadin.flow.server.VaadinService;
-import com.vaadin.flow.server.VaadinSession;
 import com.vaadin.signals.BindingActiveException;
 import com.vaadin.signals.Signal;
 import com.vaadin.signals.ValueSignal;
@@ -47,6 +50,8 @@ public class ElementBindTextTest {
 
     private MockedStatic<FeatureFlags> featureFlagStaticMock;
 
+    private LinkedList<ErrorEvent> events;
+
     @BeforeClass
     public static void init() {
         var featureFlagStaticMock = mockStatic(FeatureFlags.class);
@@ -65,15 +70,13 @@ public static void clean() {
     public void before() {
         featureFlagStaticMock = mockStatic(FeatureFlags.class);
         featureFlagEnabled(featureFlagStaticMock);
-        VaadinService.setCurrent(service);
-        VaadinSession session = new MockVaadinSession(service);
-        session.lock();
-        new MockUI(session);
+        events = mockLockedSessionWithErrorHandler();
     }
 
     @After
     public void after() {
         close(featureFlagStaticMock);
+        events = null;
     }
 
     private static void featureFlagEnabled(
@@ -91,6 +94,19 @@ private static void close(
         featureFlagStaticMock.close();
     }
 
+    private LinkedList<ErrorEvent> mockLockedSessionWithErrorHandler() {
+        VaadinService.setCurrent(service);
+
+        var session = new MockVaadinSession(service);
+        session.lock();
+
+        var ui = new MockUI(session);
+        var events = new LinkedList<ErrorEvent>();
+        session.setErrorHandler(events::add);
+
+        return events;
+    }
+
     @Test
     public void bindTextComputedSignal_getText_returnsCorrectValue() {
         Element element = new Element("span");
@@ -154,6 +170,27 @@ public void bindText_setTextWithExistingInactiveBinding_throws() {
                 () -> element.setText("text2"));
     }
 
+    @Test
+    public void bindText_initialNullSignalValue_treatAsBlank() {
+        Element element = new Element("span");
+        UI.getCurrent().getElement().appendChild(element);
+        ValueSignal<String> signal = new ValueSignal<>(String.class);
+        element.bindText(signal);
+        assertEquals("", element.getText());
+        Assert.assertTrue(events.isEmpty());
+    }
+
+    @Test
+    public void bindText_setNullSignalValue_treatAsBlank() {
+        Element element = new Element("span");
+        UI.getCurrent().getElement().appendChild(element);
+        ValueSignal<String> signal = new ValueSignal<>("text");
+        element.bindText(signal);
+        signal.value(null);
+        Assert.assertTrue(events.isEmpty());
+        assertEquals("", element.getText());
+    }
+
     @Test
     public void bindText_bindTextWithExistingActiveBinding_throws() {
         Element element = new Element("span");