Handle missing plugin name in remove command

Today if a user invokes the remove plugin command without specifying the
name of a plugin to remove, we arrive at a null pointer exception. This
commit adds logic to cleanly handle this situation and provide clear
feedback to the user.

Relates #22930

core/src/main/java/org/elasticsearch/plugins/RemovePluginCommand.java

@@ -57,6 +57,10 @@ class RemovePluginCommand extends EnvironmentAwareCommand {
 
     // pkg private for testing
     void execute(Terminal terminal, String pluginName, Environment env) throws Exception {
+        if (pluginName == null) {
+            throw new UserException(ExitCodes.USAGE, "plugin name is required");
+        }
+
         terminal.println("-> Removing " + Strings.coalesceToEmpty(pluginName) + "...");
 
         final Path pluginDir = env.pluginsFile().resolve(pluginName);

qa/evil-tests/src/test/java/org/elasticsearch/plugins/RemovePluginCommandTests.java

@@ -153,6 +153,12 @@ public class RemovePluginCommandTests extends ESTestCase {
         }
     }
 
+    public void testMissingPluginName() throws Exception {
+        UserException e = expectThrows(UserException.class, () -> removePlugin(null, home));
+        assertEquals(ExitCodes.USAGE, e.exitCode);
+        assertEquals("plugin name is required", e.getMessage());
+    }
+
     private String expectedConfigDirPreservedMessage(final Path configDir) {
         return "-> Preserving plugin config files [" + configDir + "] in case of upgrade, delete manually if not needed";
     }