do not enable nullable on netstandard2.0 (#3299)

Author: jmarolf

template_feed/Microsoft.DotNet.Common.ProjectTemplates.6.0/content/ClassLibrary-CSharp/Company.ClassLibrary1.csproj

@@ -5,7 +5,7 @@
     <TargetFramework Condition="'$(TargetFrameworkOverride)' != ''">TargetFrameworkOverride</TargetFramework>
     <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">Company.ClassLibrary1</RootNamespace>
     <LangVersion Condition="'$(langVersion)' != ''">$(ProjectLanguageVersion)</LangVersion>
-    <Nullable Condition="'$(Nullable)' == 'true'">enable</Nullable>
+    <Nullable Condition="('$(Nullable)' == 'true') AND ('$(Framework)' != 'netstandard2.0')">enable</Nullable>
   </PropertyGroup>
 
 </Project>

test/dotnet-new3.UnitTests/CommonTemplatesTests.cs

@@ -63,6 +63,9 @@ public CommonTemplatesTests(SharedHomeDirectory fixture, ITestOutputHelper log)
         [InlineData("Class Library", "classlib", "C#", "netcoreapp2.1")]
         [InlineData("Class Library", "classlib", "F#", "netcoreapp2.1")]
         [InlineData("Class Library", "classlib", "VB", "netcoreapp2.1")]
+        [InlineData("Class Library", "classlib", "C#", "netstandard2.0")]
+        [InlineData("Class Library", "classlib", "VB", "netstandard2.0")]
+        [InlineData("Class Library", "classlib", "F#", "netstandard2.0")]
 
         [InlineData("Simple Console Application", "app")]
         [InlineData("Simple Console Application", "app", "C#")]
@@ -167,6 +170,9 @@ Determining projects to restore\.\.\.
         [InlineData("Class Library", "classlib", "C#", "netcoreapp2.1")]
         [InlineData("Class Library", "classlib", "F#", "netcoreapp2.1")]
         [InlineData("Class Library", "classlib", "VB", "netcoreapp2.1")]
+        [InlineData("Class Library", "classlib", "C#", "netstandard2.0")]
+        [InlineData("Class Library", "classlib", "VB", "netstandard2.0")]
+        [InlineData("Class Library", "classlib", "F#", "netstandard2.0")]
 
         [InlineData("Simple Console Application", "app")]
         [InlineData("Simple Console Application", "app", "C#")]
@@ -306,7 +312,7 @@ public void SetPropertiesByDefault(string propertyName, string? propertyValue, s
         }
 
         [Theory]
-        //unset nullable 
+        //unset nullable
         [InlineData("Nullable", null, "--nullable", "false", "Simple Console Application", "app", null, null)]
         [InlineData("Nullable", null, "--nullable", "false", "Class Library", "classlib", null, null)]