Add null guards for Exec.ExecutionBuilder

The class `Exec.ExecutionBuilder` has three `private final` properties:
`namespace`, `name`, and `command`. While it makes little sense to
provide `null` as their values, if this happens, this would throw a
`NullPointerException` at a much later time, complicating the debugging
process.
This commit implements a fail-fast `null` guards for these variables.

Author: pingpingy1

util/src/main/java/io/kubernetes/client/Exec.java

@@ -39,6 +39,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
@@ -359,9 +360,9 @@ public final class ExecutionBuilder {
     private Consumer<Throwable> onUnhandledError;
 
     private ExecutionBuilder(String namespace, String name, String[] command) {
-      this.namespace = namespace;
-      this.name = name;
-      this.command = command;
+      this.namespace = Objects.requireNonNull(namespace, "namespace");
+      this.name = Objects.requireNonNull(name, "name");
+      this.command = Objects.requireNonNull(command, "command");
       this.stdin = true;
       this.stdout = true;
       this.stderr = true;

util/src/test/java/io/kubernetes/client/ExecTest.java

@@ -19,6 +19,7 @@
 import static com.github.tomakehurst.wiremock.client.WireMock.urlPathEqualTo;
 import static com.github.tomakehurst.wiremock.core.WireMockConfiguration.options;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.times;
@@ -281,4 +282,18 @@ public void testNonZeroBadIntExit() {
     int exitCode = Exec.parseExitCode(client, inputStream);
     assertEquals(-1, exitCode);
   }
+
+  @Test
+  public void testExecutionBuilderNull() {
+    Exec exec = new Exec(null);
+    assertThrows(NullPointerException.class, () -> {
+      exec.newExecutionBuilder(null, null, null);
+    });
+    assertThrows(NullPointerException.class, () -> {
+      exec.newExecutionBuilder("", null, null);
+    });
+    assertThrows(NullPointerException.class, () -> {
+      exec.newExecutionBuilder("", "", null);
+    });
+  }
 }