[SPARK-22130][CORE] UTF8String.trim() scans " " twice

## What changes were proposed in this pull request?

This PR allows us to scan a string including only white space (e.g. `"     "`) once while the current implementation scans twice (right to left, and then left to right).

## How was this patch tested?

Existing test suites

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #19355 from kiszk/SPARK-22130.

common/unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -498,17 +498,16 @@ private UTF8String copyUTF8String(int start, int end) {
 
   public UTF8String trim() {
     int s = 0;
-    int e = this.numBytes - 1;
     // skip all of the space (0x20) in the left side
     while (s < this.numBytes && getByte(s) == 0x20) s++;
-    // skip all of the space (0x20) in the right side
-    while (e >= 0 && getByte(e) == 0x20) e--;
-    if (s > e) {
+    if (s == this.numBytes) {
       // empty string
       return EMPTY_UTF8;
-    } else {
-      return copyUTF8String(s, e);
     }
+    // skip all of the space (0x20) in the right side
+    int e = this.numBytes - 1;
+    while (e > s && getByte(e) == 0x20) e--;
+    return copyUTF8String(s, e);
   }
 
   /**

common/unsafe/src/test/java/org/apache/spark/unsafe/types/UTF8StringSuite.java

@@ -222,10 +222,13 @@ public void substring() {
 
   @Test
   public void trims() {
+    assertEquals(fromString("1"), fromString("1").trim());
+
     assertEquals(fromString("hello"), fromString("  hello ").trim());
     assertEquals(fromString("hello "), fromString("  hello ").trimLeft());
     assertEquals(fromString("  hello"), fromString("  hello ").trimRight());
 
+    assertEquals(EMPTY_UTF8, EMPTY_UTF8.trim());
     assertEquals(EMPTY_UTF8, fromString("  ").trim());
     assertEquals(EMPTY_UTF8, fromString("  ").trimLeft());
     assertEquals(EMPTY_UTF8, fromString("  ").trimRight());