add javadoc and update tests for Strings.isBlank

Author: daniel-mohedano

internal-api/src/main/java/datadog/trace/util/Strings.java

@@ -194,6 +194,13 @@ public static boolean isNotBlank(String s) {
     return false;
   }
 
+  /**
+   * Checks that a string is blank, i.e. doest not contain characters or is null
+   *
+   * @param s The string to be checked
+   * @return {@code true} if string is blank (string is {@code null}, empty, or contains only
+   *     whitespace characters), {@code false} otherwise
+   */
   public static boolean isBlank(String s) {
     return !isNotBlank(s);
   }

internal-api/src/test/groovy/datadog/trace/util/StringsTest.groovy

@@ -117,12 +117,14 @@ class StringsTest extends DDSpecification {
     "hélló wórld" | 5     | "hélló"
   }
 
-  def "test isNotBlank: #input"() {
+  def "test isNotBlank and isBlank: #input"() {
     when:
     def notBlank = Strings.isNotBlank(input)
+    def isBlank = Strings.isBlank(input)
 
     then:
     notBlank == expected
+    isBlank == !notBlank
 
     where:
     input        | expected