Fix support for Nullable (language version 8) (#25)

* Only emit #nullable when nullable is supported (>= 8.0)

* x

* is not

src/ProxyInterfaceSourceGenerator/FileGenerators/PartialInterfacesGenerator.cs

@@ -11,8 +11,8 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
 {
     internal class PartialInterfacesGenerator : BaseGenerator, IFilesGenerator
     {
-        public PartialInterfacesGenerator(Context context) :
-            base(context)
+        public PartialInterfacesGenerator(Context context, bool supportsNullable) :
+            base(context, supportsNullable)
         {
         }
 
@@ -30,7 +30,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
             var interfaceName = targetClassSymbol.ResolveInterfaceNameWithOptionalTypeConstraints(pd.InterfaceName);
 
             var file = new FileData(
-                $"{pd.FileName}.g.cs",
+                $"{targetClassSymbol.GetFileName()}.g.cs",
                 CreatePartialInterfaceCode(pd.Namespace, targetClassSymbol, interfaceName, pd.ProxyAll)
             );
 
@@ -48,7 +48,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
 // </auto-generated>
 //----------------------------------------------------------------------------------------
 
-#nullable enable
+{(_supportsNullable ? "#nullable enable" : string.Empty)}
 using System;
 
 namespace {ns}
@@ -62,7 +62,7 @@ namespace {ns}
 {GenerateEvents(targetClassSymbol)}
     }}
 }}
-#nullable disable";
+{(_supportsNullable ? "#nullable disable" : string.Empty)}";
 
         private string GenerateProperties(INamedTypeSymbol targetClassSymbol, bool proxyAll)
         {