Add annotated string.IsNullOr* replacements

Only netstandard2.1 has annotated APIs, so code that tested for nulls using these framework methods would trigger compiler warnings on subsequent dereferences. By using this annotated version instead, the analysis operates as expected. These methods should be inlined by the JIT leaving no runtime overhead.

Author: drewnoakes

src/NetMQ/Utils/Strings.cs

@@ -0,0 +1,15 @@
+#nullable enable
+
+using System.Diagnostics.CodeAnalysis;
+
+namespace NetMQ
+{
+    internal static class Strings
+    {
+        /// <inheritdoc cref="string.IsNullOrEmpty(string)"/>
+        public static bool IsNullOrEmpty([NotNullWhen(returnValue: false)] string? s) => string.IsNullOrEmpty(s);
+
+        /// <inheritdoc cref="string.IsNullOrWhiteSpace(string)"/>
+        public static bool IsNullOrWhiteSpace([NotNullWhen(returnValue: false)] string? s) => string.IsNullOrWhiteSpace(s);
+    }
+}