Disable nullability checks for older target frameworks

CliFx/CliFx.csproj

@@ -18,6 +18,11 @@
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
   </PropertyGroup>
 
+  <!-- Disable nullability warnings on older frameworks because there is no nullability info for BCL -->
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard2.0' or '$(TargetFramework)' == 'net45'">
+    <Nullable>annotations</Nullable>
+  </PropertyGroup>
+
   <ItemGroup>
     <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
       <_Parameter1>$(AssemblyName).Tests</_Parameter1>

CliFx/Domain/CommandSchema.cs

@@ -210,8 +210,8 @@ namespace CliFx.Domain
                 type,
                 attribute?.Name,
                 attribute?.Description,
-                parameters,
-                options
+                parameters!,
+                options!
             );
         }
     }

CliFx/Domain/HelpTextWriter.cs

@@ -279,9 +279,9 @@ namespace CliFx.Domain
                 foreach (var childCommand in childCommands)
                 {
                     var relativeCommandName =
-                        string.IsNullOrWhiteSpace(childCommand.Name) || string.IsNullOrWhiteSpace(command.Name)
-                            ? childCommand.Name
-                            : childCommand.Name.Substring(command.Name.Length + 1);
+                        !string.IsNullOrWhiteSpace(command.Name)
+                            ? childCommand.Name!.Substring(command.Name.Length + 1)
+                            : childCommand.Name!;
 
                     // Name
                     RenderIndent();