Issue 1596: Runner throws null ref exception when new line after EOF is missing (#1687)

* Issue 1596: runner throws nullref exception when writting env var

* Adding tests for missing new line after EOF marker

* Changing newline to new line

src/Runner.Worker/FileCommandManager.cs

@@ -181,6 +181,10 @@ namespace GitHub.Runner.Worker
                                 {
                                     throw new Exception($"Invalid environment variable value. Matching delimiter not found '{delimiter}'");
                                 }
+                                if (newline == null)
+                                {
+                                    throw new Exception($"Invalid environment variable value. EOF marker missing new line.");
+                                }
                                 endIndex = index - newline.Length;
                                 tempLine = ReadLine(text, ref index, out newline);
                             }