fixed DebugType ArgumentNullException

in the situation where DebugType is mising and the DebugSymbols property is set to true, the TryGetValue method of the s_debugTypeValues dictionary would throw an exception

src/Workspaces/Core/MSBuild/MSBuild/ProjectFile/CommandLineArgumentReader.cs

@@ -167,8 +167,7 @@ protected void ReadDebugInfo()
             if (emitDebugInfo)
             {
                 var debugType = Project.ReadPropertyString(PropertyNames.DebugType);
-
-                if (s_debugTypeValues.TryGetValue(debugType, out var value))
+                if (debugType != null && s_debugTypeValues.TryGetValue(debugType, out var value))
                 {
                     Add("debug", value);
                 }