Add spotbug checks for pulsar-config-validation module (apache#9034)

### Motivation

Add spotbug checks for pulsar-config-validation module.

### Modifications

The only failure check is the redundant null check, so we just remove the null check.

### Verifying this change

- [ ] Make sure that the change passes the CI checks.

*(Please pick either of the following options)*

This change is a trivial rework / code cleanup without any test coverage.

pulsar-config-validation/pom.xml

@@ -39,4 +39,14 @@
             <artifactId>slf4j-api</artifactId>
         </dependency>
     </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>com.github.spotbugs</groupId>
+                <artifactId>spotbugs-maven-plugin</artifactId>
+                <version>${spotbugs-maven-plugin.version}</version>
+            </plugin>
+        </plugins>
+    </build>
 </project>

pulsar-config-validation/src/main/java/org/apache/pulsar/config/validation/ConfigValidationUtils.java

@@ -90,8 +90,7 @@ public void validateField(String pd, String name, Object field)
                     return;
                 }
                 throw new IllegalArgumentException(
-                        "Field " + name + " must be an Iterable but was "
-                                + ((field == null) ? "null" : ("a " + field.getClass())));
+                        "Field " + name + " must be an Iterable but was a " + field.getClass());
             }
         };
     }