[SPARK-51107][CORE] Refactor CommandBuilderUtils#join to reuse the li…

…nes about strings join and reduce the redundant lines

### What changes were proposed in this pull request?
- Refactor CommandBuilderUtils#join to reuse the lines about strings join and reduce the redundant lines.

### Why are the changes needed?

- Refactor CommandBuilderUtils#join to reuse the lines about strings join and reduce the redundant lines.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Pass GitHub Actions

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #49826 from RocMarshal/SPARK-51107.

Authored-by: Roc Marshal <[email protected]>
Signed-off-by: yangjie01 <[email protected]>

launcher/src/main/java/org/apache/spark/launcher/CommandBuilderUtils.java

@@ -19,6 +19,7 @@
 
 import java.io.File;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.regex.Matcher;
@@ -46,24 +47,15 @@ static boolean isEmpty(String s) {
 
   /** Joins a list of strings using the given separator. */
   static String join(String sep, String... elements) {
-    StringBuilder sb = new StringBuilder();
-    for (String e : elements) {
-      if (e != null) {
-        if (sb.length() > 0) {
-          sb.append(sep);
-        }
-        sb.append(e);
-      }
-    }
-    return sb.toString();
+    return join(sep, Arrays.asList(elements));
   }
 
   /** Joins a list of strings using the given separator. */
   static String join(String sep, Iterable<String> elements) {
     StringBuilder sb = new StringBuilder();
     for (String e : elements) {
       if (e != null) {
-        if (sb.length() > 0) {
+        if (!sb.isEmpty()) {
           sb.append(sep);
         }
         sb.append(e);