Added validation for null and empty domain names

Author: Lightwood13

src/main/java/org/prebid/server/util/HttpUtil.java

@@ -125,6 +125,12 @@ private static boolean containsMacrosses(String url) {
     }
 
     public static String validateDomainName(String domainName) {
+        if (domainName == null) {
+            throw new PreBidException("Domain name is null");
+        }
+        if (domainName.isEmpty()) {
+            return domainName;
+        }
         if (!VALID_DOMAIN.matcher(domainName).matches()) {
             throw new PreBidException("Domain name %s contains invalid characters".formatted(domainName));
         }

src/test/java/org/prebid/server/util/HttpUtilTest.java

@@ -71,6 +71,15 @@ public void validateDomainNameShouldReturnExpectedDomainName() {
         assertThat(HttpUtil.validateDomainName("sub.domain-example.com")).isEqualTo("sub.domain-example.com");
         assertThat(HttpUtil.validateDomainName("127.0.0.1")).isEqualTo("127.0.0.1");
         assertThat(HttpUtil.validateDomainName("example.com:8080")).isEqualTo("example.com:8080");
+        assertThat(HttpUtil.validateDomainName("")).isEqualTo("");
+    }
+
+    @Test
+    public void validateDomainNameShouldFailOnNull() {
+        // when and then
+        assertThatExceptionOfType(PreBidException.class)
+                .isThrownBy(() -> HttpUtil.validateDomainName(null))
+                .withMessage("Domain name is null");
     }
 
     @Test