Enable nullability annotations in the project

Author: Numpsy

src/Serilog.Enrichers.Environment/Enrichers/CachedPropertyEnricher.cs

@@ -19,7 +19,7 @@ namespace Serilog.Enrichers
 {
     public abstract class CachedPropertyEnricher: ILogEventEnricher
     {
-        private LogEventProperty _cachedProperty { get; set; }
+        private LogEventProperty? _cachedProperty { get; set; }
 
         /// <summary>
         /// Enrich the log event.
@@ -35,8 +35,7 @@ private LogEventProperty GetLogEventProperty(ILogEventPropertyFactory propertyFa
         {
             // Don't care about thread-safety, in the worst case the field gets overwritten and one
             // property will be GCed
-            if (_cachedProperty == null)
-                _cachedProperty = CreateProperty(propertyFactory);
+            _cachedProperty ??= CreateProperty(propertyFactory);
 
             return _cachedProperty;
         }

src/Serilog.Enrichers.Environment/Enrichers/EnvironmentVariableEnricher.cs

@@ -31,7 +31,7 @@ public class EnvironmentVariableEnricher : CachedPropertyEnricher
         /// </summary>
         public string EnvironmentVariablePropertyName { get; }
 
-        public EnvironmentVariableEnricher(string envVarName, string propertyName)
+        public EnvironmentVariableEnricher(string envVarName, string? propertyName)
         {
             _envVarName = envVarName;
             EnvironmentVariablePropertyName = propertyName ?? envVarName;

src/Serilog.Enrichers.Environment/EnvironmentLoggerConfigurationExtensions.cs

@@ -69,7 +69,7 @@ public static LoggerConfiguration WithEnvironmentUserName(
         /// <param name="propertyName">The Optional name of the property. If empty <see cref="environmentVariableName"/> is used</param>
         /// <returns>Configuration object allowing method chaining.</returns>
         public static LoggerConfiguration WithEnvironmentVariable(
-            this LoggerEnrichmentConfiguration enrichmentConfiguration, string environmentVariableName, string propertyName = null)
+            this LoggerEnrichmentConfiguration enrichmentConfiguration, string environmentVariableName, string? propertyName = null)
         {
             if (enrichmentConfiguration == null) throw new ArgumentNullException(nameof(enrichmentConfiguration));
             var environmentVariableEnricher = new EnvironmentVariableEnricher(environmentVariableName, propertyName);

src/Serilog.Enrichers.Environment/Serilog.Enrichers.Environment.csproj

@@ -17,6 +17,8 @@
     <RepositoryUrl>https://github.com/serilog/serilog-enrichers-environment</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
     <GenerateAssemblyVersionAttribute>false</GenerateAssemblyVersionAttribute>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>