chore: enable nullable for SymbolResolver.cs

Author: filzrev

src/BenchmarkDotNet/Disassemblers/Exporters/SymbolResolver.cs

@@ -1,6 +1,8 @@
 using Iced.Intel;
 using System.Collections.Generic;
 
+#nullable enable
+
 namespace BenchmarkDotNet.Disassemblers.Exporters
 {
     internal sealed class SymbolResolver : ISymbolResolver
@@ -11,7 +13,7 @@ internal sealed class SymbolResolver : ISymbolResolver
 
         public bool TryGetSymbol(in Instruction instruction, int operand, int instructionOperand, ulong address, int addressSize, out SymbolResult symbol)
         {
-            if (_mappings.TryGetValue(address, out string text))
+            if (_mappings.TryGetValue(address, out var text))
             {
                 symbol = new SymbolResult(address, text);
                 return true;