Add final var (#813)

Remove redundant String.

src/main/java/org/apache/commons/lang3/CharUtils.java

@@ -303,7 +303,7 @@ public class CharUtils {
         if (ch < CHAR_STRING_ARRAY.length) {
             return CHAR_STRING_ARRAY[ch];
         }
-        return new String(new char[] {ch});
+        return String.valueOf(ch);
     }
 
     /**

src/main/java/org/apache/commons/lang3/ThreadUtils.java

@@ -467,7 +467,7 @@ public class ThreadUtils {
     public static void sleepQuietly(final long millis) {
         try {
             sleep(Duration.ofMillis(millis));
-        } catch (InterruptedException e) {
+        } catch (final InterruptedException e) {
             // be quiet.
         }
     }