Edited Issue: Queries broken on providers that return null for DbConnection.DataSource [1164]

The System.Data.SQLite provider returns null for `DbConnection.DataSource`. During query execution, we use this value to lookup an execution strategy, and check that it is not null or empty inside ExecutionStrategyKey's constructor.

__Exception:__
```
System.ArgumentException: The argument 'serverName' cannot be null, empty or contain only white space.
at System.Data.Entity.Utilities.Check.NotEmpty(String value, String parameterName)
at System.Data.Entity.Infrastructure.ExecutionStrategyKey..ctor(String providerInvariantName, String serverName)
at System.Data.Entity.Core.Common.DbProviderServices.GetExecutionStrategy(DbConnection connection, DbProviderFactory providerFactory)
at System.Data.Entity.Core.Common.DbProviderServices.GetExecutionStrategy(DbConnection connection, MetadataWorkspace metadataWorkspace)
at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0()
at System.Lazy`1.CreateValue()
at System.Lazy`1.LazyInitValue()
at System.Lazy`1.get_Value()
at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
```