Test and fix for #591

This brings behavior QueryAsync behavior on par with Query (sync) and properly returns nothing when an empty result set is encountered.

Dapper.Tests/ProcedureTests.cs

 using System;
 using System.Data;
 using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Dapper.Tests
@@ -219,7 +220,7 @@ select DATEADD(ns, -100, @b)
             Assert.Equal(datetime2, p.Get<DateTime>("b"));
         }
 
-        [Theory()]
+        [Theory]
         [InlineData(null)]
         [InlineData(DbType.DateTime)]
         public void TestDateTime2LosePrecisionInDynamicParameters(DbType? dbType)
@@ -254,5 +255,27 @@ select @b
             // @b gets set to datetime2 value but is truncated back to DbType.DateTime by DynamicParameter's Output declaration
             Assert.Equal(datetimeDefault, p.Get<DateTime>("b"));
         }
+
+
+        [Fact]
+        public async Task Issue591_NoResultsAsync()
+        {
+            const string tempSPName = "#" + nameof(Issue591_NoResultsAsync);
+
+            var result = await connection.QueryAsync(
+            $@"create proc {tempSPName}
+            as 
+            begin
+                -- basically a failed if statement, so the select is not happening and the stored proc return nothing
+                if 1=0
+                begin
+                    select 1 as Num
+                end
+            end
+            
+            exec {tempSPName}");
+
+            Assert.Empty(result);
+        }
     }
 }

Dapper/SqlMapper.Async.cs

@@ -392,6 +392,8 @@ private static async Task<IEnumerable<T>> QueryAsync<T>(this IDbConnection cnn,
                     int hash = GetColumnHash(reader);
                     if (tuple.Func == null || tuple.Hash != hash)
                     {
+                        if (reader.FieldCount == 0)
+                            return Enumerable.Empty<T>();
                         tuple = info.Deserializer = new DeserializerState(hash, GetDeserializer(effectiveType, reader, 0, -1, false));
                         if (command.AddToCache) SetQueryCache(identity, info);
                     }