Minor: Rename StringUtils#isEmpty to #isBlank

- Better fits the naming used by other tools and also the JDK (String#isBlank as of JDK 11)

src/main/java/fr/xephi/authme/command/CommandDescription.java

@@ -226,8 +226,8 @@ public class CommandDescription {
          */
         public CommandDescription build() {
             checkArgument(!Utils.isCollectionEmpty(labels), "Labels may not be empty");
-            checkArgument(!StringUtils.isEmpty(description), "Description may not be empty");
-            checkArgument(!StringUtils.isEmpty(detailedDescription), "Detailed description may not be empty");
+            checkArgument(!StringUtils.isBlank(description), "Description may not be empty");
+            checkArgument(!StringUtils.isBlank(detailedDescription), "Detailed description may not be empty");
             checkArgument(executableCommand != null, "Executable command must be set");
             // parents and permissions may be null; arguments may be empty
 

src/main/java/fr/xephi/authme/command/CommandHandler.java

@@ -131,7 +131,7 @@ public class CommandHandler {
     private static List<String> skipEmptyArguments(String[] args) {
         List<String> cleanArguments = new ArrayList<>();
         for (String argument : args) {
-            if (!StringUtils.isEmpty(argument)) {
+            if (!StringUtils.isBlank(argument)) {
                 cleanArguments.add(argument);
             }
         }

src/main/java/fr/xephi/authme/command/executable/authme/debug/PlayerAuthViewer.java

@@ -89,7 +89,7 @@ class PlayerAuthViewer implements DebugSection {
      *         or empty string if the string is null or empty
      */
     private static String safeSubstring(String str, int length) {
-        if (StringUtils.isEmpty(str)) {
+        if (StringUtils.isBlank(str)) {
             return "";
         } else if (str.length() < length) {
             return str.substring(0, str.length() / 2) + "...";