Remove `Required` constraints of `EntityChange's EntityId`.

modules/audit-logging/src/Volo.Abp.AuditLogging.EntityFrameworkCore/Volo/Abp/AuditLogging/EntityFrameworkCore/AbpAuditLoggingDbContextModelBuilderExtensions.cs

@@ -73,7 +73,7 @@ public static class AbpAuditLoggingDbContextModelBuilderExtensions
             b.ConfigureByConvention();
 
             b.Property(x => x.EntityTypeFullName).HasMaxLength(EntityChangeConsts.MaxEntityTypeFullNameLength).IsRequired().HasColumnName(nameof(EntityChange.EntityTypeFullName));
-            b.Property(x => x.EntityId).HasMaxLength(EntityChangeConsts.MaxEntityIdLength).IsRequired().HasColumnName(nameof(EntityChange.EntityId));
+            b.Property(x => x.EntityId).HasMaxLength(EntityChangeConsts.MaxEntityIdLength).HasColumnName(nameof(EntityChange.EntityId));
             b.Property(x => x.AuditLogId).IsRequired().HasColumnName(nameof(EntityChange.AuditLogId));
             b.Property(x => x.ChangeTime).IsRequired().HasColumnName(nameof(EntityChange.ChangeTime));
             b.Property(x => x.ChangeType).IsRequired().HasColumnName(nameof(EntityChange.ChangeType));