fix: scaladoc link and coverage of empty checkpoint properties validation

The `@throws IllegalArgumentException` tag could not be resolved by Scaladoc,
failing `sbt doc` under -Xfatal-warnings; the exception is now described in the
`properties` parameter doc. Replaced the partial-function `collect` with `filter`
to avoid untestable generated bytecode lowering server coverage below threshold.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lsulak

reader/src/main/scala/za/co/absa/atum/reader/requests/CheckpointFilter.scala

@@ -34,8 +34,7 @@ import za.co.absa.atum.model.utils.JsonSyntaxExtensions._
  *  @param name       - only checkpoints of this name
  *  @param properties - only checkpoints that, for every property name given, have that property with one of the given
  *                      values, e.g. `Map("executionID" -> Set("a", "b"))` means `executionID IN (a, b)`; every property
- *                      needs at least one value
- *  @throws IllegalArgumentException if a property is given without any value
+ *                      needs at least one value, otherwise an `IllegalArgumentException` is thrown
  */
 case class CheckpointFilter(
   name: Option[String] = None,
@@ -44,7 +43,7 @@ case class CheckpointFilter(
 
   require(
     properties.values.forall(_.nonEmpty),
-    s"Checkpoint properties without any accepted value: ${properties.collect { case (n, v) if v.isEmpty => n }.mkString(", ")}"
+    s"Checkpoint properties without any accepted value: ${properties.filter(_._2.isEmpty).keys.mkString(", ")}"
   )
 
   private[reader] def toQueryParams: Map[String, String] = {

server/src/main/scala/za/co/absa/atum/server/api/v2/http/Endpoints.scala

@@ -50,7 +50,7 @@ object Endpoints extends BaseEndpoints {
       }
     }(_.asBase64EncodedJsonString)
       .validate(Validator.custom { properties =>
-        val propertiesWithoutValues = properties.collect { case (name, values) if values.isEmpty => name }
+        val propertiesWithoutValues = properties.filter(_._2.isEmpty).keys
         if (propertiesWithoutValues.isEmpty) ValidationResult.Valid
         else
           ValidationResult.Invalid(