Fixed ExpressionParser when WrappedValue-string is used for equals-operator (#666)

* UT

* ?

* Fixed ExpressionParser when WrappedValue-string is used for equals-operator

Author: StefH

src/System.Linq.Dynamic.Core/Compatibility/Nullable/NotNullWhenAttribute.cs

@@ -0,0 +1,56 @@
+#region License
+// MIT License
+// 
+// Copyright (c) Manuel Römer
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+#endregion
+
+#if NETSTANDARD1_3_OR_GREATER || NET35 || NET40 || NET45 || NET452 || NET46 || NETCOREAPP2_1 || UAP10_0
+
+// ReSharper disable once CheckNamespace
+namespace System.Diagnostics.CodeAnalysis;
+
+/// <summary>
+/// Specifies that when a method returns <see cref="ReturnValue"/>,
+/// the parameter will not be <see langword="null"/> even if the corresponding type allows it.
+/// </summary>
+[AttributeUsage(AttributeTargets.Parameter)]
+[DebuggerNonUserCode]
+internal sealed class NotNullWhenAttribute : Attribute
+{
+    /// <summary>
+    /// Gets the return value condition.
+    /// If the method returns this value, the associated parameter will not be <see langword="null"/>.
+    /// </summary>
+    public bool ReturnValue { get; }
+
+    /// <summary>
+    /// Initializes the attribute with the specified return value condition.
+    /// </summary>
+    /// <param name="returnValue">
+    /// The return value condition.
+    /// If the method returns this value, the associated parameter will not be <see langword="null"/>.
+    /// </param>
+    public NotNullWhenAttribute(bool returnValue)
+    {
+        ReturnValue = returnValue;
+    }
+}
+#endif