Fixed issue where build fails due to release notes

Author: Jake Ginnivan

build.cake

@@ -131,8 +131,9 @@ Task("Create-Release-Notes")
         var releaseNotesExitCode = StartProcess(
                 @"tools\GitReleaseNotes\tools\gitreleasenotes.exe",
                 new ProcessSettings { Arguments = ". /o build/releasenotes.md /repoToken " + githubToken });
-        if (string.IsNullOrEmpty(System.IO.File.ReadAllText("./build/releasenotes.md")))
+        if (!System.IO.File.Exists("./build/releasenotes.md") || string.IsNullOrEmpty(System.IO.File.ReadAllText("./build/releasenotes.md"))) {
             System.IO.File.WriteAllText("./build/releasenotes.md", "No issues closed since last release");
+        }
 
         if (releaseNotesExitCode != 0) throw new Exception("Failed to generate release notes");
     }