fix: prevent NPE when creating init parameters (#19856) (#19857)

ServletConfig might contain init parameter with null values. This is however not
supported by java Properties class.
This change adds a null check and logs offending keys for debugging purpose.

Fixes #19855

Co-authored-by: Marco Collovati <marco@vaadin.com>

flow-server/src/main/java/com/vaadin/flow/server/DeploymentConfigurationFactory.java

@@ -21,6 +21,8 @@
 import java.util.Map;
 import java.util.Properties;
 
+import org.slf4j.LoggerFactory;
+
 import com.vaadin.flow.component.UI;
 import com.vaadin.flow.function.DeploymentConfiguration;
 import com.vaadin.flow.server.startup.AbstractConfigurationFactory;
@@ -94,8 +96,13 @@ protected Properties createInitParameters(Class<?> systemPropertyBaseClass,
         for (final Enumeration<String> e = vaadinConfig
                 .getConfigParameterNames(); e.hasMoreElements();) {
             final String name = e.nextElement();
-            initParameters.setProperty(name,
-                    vaadinConfig.getConfigParameter(name));
+            String value = vaadinConfig.getConfigParameter(name);
+            if (value != null) {
+                initParameters.setProperty(name, value);
+            } else {
+                LoggerFactory.getLogger(DeploymentConfigurationFactory.class)
+                        .debug("Ignoring NULL init parameter {}", name);
+            }
         }
 
         readBuildInfo(initParameters, vaadinConfig.getVaadinContext());

flow-server/src/test/java/com/vaadin/flow/server/DeploymentConfigurationFactoryTest.java

@@ -187,6 +187,29 @@ public void servletConfigParametersOverrideServletContextParameters()
                 config.getHeartbeatInterval());
     }
 
+    @Test
+    public void servletConfigParameters_nullValues_ignored() throws Exception {
+        Class<NoSettings> servlet = NoSettings.class;
+
+        Map<String, String> servletConfigParams = new HashMap<>(
+                defaultServletParams);
+        servletConfigParams.put("someKey", null);
+        servletConfigParams.put("someNotNullKey", "NOT_NULL");
+
+        Map<String, String> servletContextParams = new HashMap<>();
+
+        DeploymentConfiguration config = new DeploymentConfigurationFactory()
+                .createDeploymentConfiguration(servlet, createVaadinConfigMock(
+                        servletConfigParams, servletContextParams));
+
+        Assert.assertFalse(
+                "Expecting null parameter to be ignored, but was in configuration",
+                config.getInitParameters().containsKey("someKey"));
+        Assert.assertTrue(
+                "Expecting not null parameter to be in configuration, but was not",
+                config.getInitParameters().containsKey("someNotNullKey"));
+    }
+
     @Test
     public void should_readConfigurationFromTokenFile() throws Exception {
         FileUtils.writeLines(tokenFile,