null check for text in messages of TestResult and print it on console if its not null or empty.

[Faizan2304]

Issue: #801

[msftclas]

@Faizan2304,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot

[Faizan2304]

For following tests

using NUnit.Framework;
using System;
namespace nunit24214asf
{
	[TestFixture()]
	public class Test
	{
		[Test()]
		public void TestCase()
		{
			Assert.AreEqual(3, 4);
		}
	}
}

Without Fix output would be:

Failed   TestCase
Error Message:
   Expected: 3
  But was:  4

Stack Trace:
at nunit24214asf.Test.TestCase() in c:\users\faahmad\documents\visual studio 2017\Projects\UnitTestProject1\UnitTestProject1\UnitTest
1.cs:line 11

Standard Output Messages:

Total tests: 1. Passed: 0. Failed: 1. Skipped: 0.
Test Run Failed.

### With Fix output would be:

Failed   TestCase
Error Message:
   Expected: 3
  But was:  4

Stack Trace:
at nunit24214asf.Test.TestCase() in c:\users\faahmad\documents\visual studio 2017\Projects\UnitTestProject1\UnitTestProject1\UnitTest
1.cs:line 11


Total tests: 1. Passed: 0. Failed: 1. Skipped: 0.
Test Run Failed.

[Faizan2304]

@dotnet-bot test Windows_NT / Debug Build please

[Faizan2304]

@dotnet-bot test Windows_NT / Debug Build please