Fix NRE thrown when version property is missing

Fixes #250

Author: AArnott

src/NerdBank.GitVersioning/VersionOracle.cs

@@ -98,7 +98,7 @@ public VersionOracle(string projectDirectory, LibGit2Sharp.Repository repo, LibG
 
             this.VersionHeightOffset = this.VersionOptions?.BuildNumberOffsetOrDefault ?? 0;
 
-            this.PrereleaseVersion = this.ReplaceMacros(this.VersionOptions?.Version.Prerelease ?? string.Empty);
+            this.PrereleaseVersion = this.ReplaceMacros(this.VersionOptions?.Version?.Prerelease ?? string.Empty);
 
             this.CloudBuildNumberOptions = this.VersionOptions?.CloudBuild?.BuildNumberOrDefault ?? VersionOptions.CloudBuildNumberOptions.DefaultInstance;