Refactor ReadReceiverNameQuery and improve validation

- Removed parameters from ReadReceiverNameQuery, simplifying its structure.
- Added null check for EmailAddress in GetReceiverName method to enhance input validation.

EnvelopeGenerator.GeneratorAPI/Controllers/EnvelopeReceiverController.cs

@@ -125,6 +125,9 @@ public class EnvelopeReceiverController : ControllerBase
     [HttpGet("salute")]
     public async Task<IActionResult> GetReceiverName([FromQuery] ReadReceiverNameQuery receiverName)
     {
+        if (receiverName.EmailAddress is null)
+            return BadRequest();
+
         return await _erService.ReadLastUsedReceiverNameByMail(receiverName.EmailAddress).ThenAsync(
                         Success: res => res is null ? Ok(string.Empty) : Ok(res),
                         Fail: IActionResult (msg, ntc) =>