Build failed if DefineConstants is empty.

[mfilippov]

If fsproj file not contains DefineConstants element. Build task failed with NullReferenceException.

Using task Fsc from Microsoft.FSharp.Build.Fsc, FSharp.Build, Version=4.4.0.0, Culture=neutral, PublicKeyToken=f536804aa0eb945b
/usr/local/Cellar/mono/4.6.1.3/lib/mono/fsharp/Microsoft.FSharp.Targets: error : Error executing task Fsc: Object reference not set to an instance of an object
        Error executing task Fsc: System.NullReferenceException: Object reference not set to an instance of an object
          at Microsoft.FSharp.Build.Fsc.generateCommandLineBuilder () [0x0012c] in <580520ea804cd226a7450383ea200558>:0 
          at Microsoft.FSharp.Build.Fsc.GenerateResponseFileCommands () [0x00016] in <580520ea804cd226a7450383ea200558>:0 
          at Microsoft.Build.Utilities.ToolTask.Execute () [0x0001c] in <354d8a08a69244418011ac90fd434d2b>:0 
          at Microsoft.Build.BuildEngine.TaskEngine.Execute () [0x00000] in <29ed228a0f8e4d039e4eda81cff6d54b>:0 
          at Microsoft.Build.BuildEngine.BuildTask.Execute () [0x0008f] in <29ed228a0f8e4d039e4eda81cff6d54b>:0 

[dsyme]

This should now be fixed. Could you check please? @kumpera @ony @mfilippov

[dsyme]

Closing as this is, we believe, fixed, but please reopen if needed