refactor(ReadEnvelopeReceiverQuery): add receiver filter to signatures

EnvelopeGenerator.Application/Common/Query/ReceiverQueryBase.cs

@@ -20,4 +20,14 @@ public record ReceiverQueryBase
     /// Eindeutige Signatur des Empfängers
     /// </summary>
     public virtual string? Signature { get; set; }
+
+    /// <summary>
+    /// Checks whether any of the specified query criteria have a value.
+    /// </summary>
+    /// <remarks>
+    /// This property returns <c>true</c> if at least one of the fields 
+    /// <see cref="Id"/>, <see cref="EmailAddress"/>, or <see cref="Signature"/> is not null.
+    /// <para>Usage example: The query can be executed only if at least one criterion is specified.</para>
+    /// </remarks>
+    public bool HasAnyCriteria => Id is not null || EmailAddress is not null || Signature is not null;
 }