fixed validation in transaction creation

transaction-service/src/main/java/cz/muni/fi/obs/api/TransactionCreateDto.java

@@ -2,6 +2,7 @@ package cz.muni.fi.obs.api;
 
 import jakarta.validation.constraints.Min;
 import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotNull;
 import lombok.Builder;
 
 import java.math.BigDecimal;
@@ -12,7 +13,8 @@ public record TransactionCreateDto(
 		String withdrawsFromAccountNumber,
 		@NotBlank
 		String depositsToAccountNumber,
-		@Min(0) BigDecimal withdrawAmount,
+		@Min(0)
+		@NotNull BigDecimal withdrawAmount,
 		String note,
 		String variableSymbol) {
 }