[HUDI-615]: Add some methods and test cases for StringUtils. (#1338)

hudi-common/src/main/java/org/apache/hudi/common/util/StringUtils.java

@@ -18,6 +18,8 @@
 
 package org.apache.hudi.common.util;
 
+import javax.annotation.Nullable;
+
 /**
  * Simple utility for operations on strings.
  */
@@ -67,4 +69,29 @@ public class StringUtils {
   public static boolean isNullOrEmpty(String str) {
     return str == null || str.length() == 0;
   }
+
+
+  /**
+   * Returns the given string if it is non-null; the empty string otherwise.
+   *
+   * @param string the string to test and possibly return
+   * @return {@code string} itself if it is non-null; {@code ""} if it is null
+   */
+  public static String nullToEmpty(@Nullable String string) {
+    return string == null ? "" : string;
+  }
+
+  /**
+   * Returns the given string if it is nonempty; {@code null} otherwise.
+   *
+   * @param string the string to test and possibly return
+   * @return {@code string} itself if it is nonempty; {@code null} if it is empty or null
+   */
+  public static @Nullable String emptyToNull(@Nullable String string) {
+    return stringIsNullOrEmpty(string) ? null : string;
+  }
+
+  private static boolean stringIsNullOrEmpty(@Nullable String string) {
+    return string == null || string.isEmpty();
+  }
 }