Apply nullability

AdventOfCode2017/AdventOfCode2017.csproj

@@ -4,6 +4,7 @@
         <OutputType>Exe</OutputType>
         <TargetFramework>net8.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
+        <Nullable>enable</Nullable>
         <RootNamespace>AdventOfCode2017</RootNamespace>
     </PropertyGroup>
 

AdventOfCode2017/Day03.cs

@@ -30,11 +30,11 @@ public class Day03 : IDay
 {
     public string ResolvePart1(string[] inputs)
     {
-        return null;
+        return string.Empty;
     }
 
     public string ResolvePart2(string[] inputs)
     {
-        return null;
+        return string.Empty;
     }
 }