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

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

* x

* is not

src/ProxyInterfaceSourceGenerator/FileGenerators/ProxyClassesGenerator.cs

@@ -12,7 +12,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
 {
     internal class ProxyClassesGenerator : BaseGenerator, IFilesGenerator
     {
-        public ProxyClassesGenerator(Context context) : base(context)
+        public ProxyClassesGenerator(Context context, bool supportsNullable) : base(context, supportsNullable)
         {
         }
 
@@ -32,7 +32,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
             var constructorName = $"{targetClassSymbol.Name}Proxy";
 
             var file = new FileData(
-                $"{pd.FileName}Proxy.g.cs",
+                $"{targetClassSymbol.GetFileName()}Proxy.g.cs",
                 CreateProxyClassCode(pd.Namespace, targetClassSymbol, interfaceName, className, constructorName)
             );
 
@@ -41,7 +41,12 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
             return file;
         }
 
-        private string CreateProxyClassCode(string ns, INamedTypeSymbol targetClassSymbol, string interfaceName, string className, string constructorName) => $@"//----------------------------------------------------------------------------------------
+        private string CreateProxyClassCode(
+            string ns,
+            INamedTypeSymbol targetClassSymbol,
+            string interfaceName,
+            string className,
+            string constructorName) => $@"//----------------------------------------------------------------------------------------
 // <auto-generated>
 //     This code was generated by https://github.com/StefH/ProxyInterfaceSourceGenerator.
 //
@@ -50,7 +55,7 @@ namespace ProxyInterfaceSourceGenerator.FileGenerators
 // </auto-generated>
 //----------------------------------------------------------------------------------------
 
-#nullable enable
+{(_supportsNullable ? "#nullable enable" : string.Empty)}
 using System;
 using AutoMapper;
 
@@ -76,7 +81,7 @@ namespace {ns}
 {GeneratePrivateAutoMapper()}
     }}
 }}
-#nullable disable";
+{(_supportsNullable ? "#nullable disable" : string.Empty)}";
         private string GeneratePrivateAutoMapper()
         {
             return _context.ReplacedTypes.Count == 0 ? string.Empty : "        private readonly IMapper _mapper;";