Fixes for unit tests

PkmnLib.Static/Utils/StringKey.cs

@@ -1,3 +1,5 @@
+using System.Diagnostics.CodeAnalysis;
+
 namespace PkmnLib.Static.Utils;
 
 /// <summary>
@@ -26,8 +28,18 @@ public readonly record struct StringKey
     /// <summary>
     /// Converts a <see cref="string"/> to a <see cref="StringKey"/>.
     /// </summary>
+    [return: NotNullIfNotNull("key")]
+    public static implicit operator StringKey?(string? key) =>
+        string.IsNullOrWhiteSpace(key) ? null! : new StringKey(key);
+
+    /// <summary>
+    /// Converts a <see cref="string"/> to a <see cref="StringKey"/>.
+    /// Throws an <see cref="ArgumentException"/> if the key is null or whitespace.
+    /// </summary>
     public static implicit operator StringKey(string key) =>
-        string.IsNullOrWhiteSpace(key) ? default : new StringKey(key);
+        string.IsNullOrWhiteSpace(key)
+            ? throw new ArgumentException("Key cannot be null or whitespace.", nameof(key))
+            : new StringKey(key);
 
     /// <inheritdoc cref="string.ToString()"/>
     public override string ToString() => _key.ToLowerInvariant();