Fix first set of code review comments

Author: tamasvajk

csharp/extractor/Semmle.Extraction.CIL/Entities/File.cs

@@ -37,7 +37,7 @@ public override IEnumerable<IExtractionProduct> Contents
             get
             {
                 var directoryName = System.IO.Path.GetDirectoryName(path);
-                if (directoryName == null)
+                if (directoryName is null)
                     throw new InternalError($"Directory name for path '{path}' is null.");
 
                 var parent = cx.CreateFolder(directoryName);

csharp/extractor/Semmle.Extraction.CIL/Entities/Method.cs

@@ -146,7 +146,7 @@ sealed class DefinitionMethod : Method, IMember
         readonly PDB.IMethod? methodDebugInformation;
         readonly Type declaringType;
 
-        string name;
+        readonly string name;
         LocalVariable[]? locals;
 
         public MethodImplementation? Implementation { get; private set; }
@@ -417,7 +417,7 @@ public MemberReferenceMethod(GenericContext gc, MemberReferenceHandle handle) :
 
             var parentMethod = parent as Method;
 
-            var declType = parentMethod == null ? parent as Type : parentMethod.DeclaringType;
+            var declType = parentMethod is null ? parent as Type : parentMethod.DeclaringType;
 
             if (declType is null)
                 throw new InternalError("Parent context of method is not a type");

csharp/extractor/Semmle.Extraction.CIL/Entities/Namespace.cs

@@ -20,7 +20,7 @@ public sealed class Namespace : TypeContainer, INamespace
         public Namespace? ParentNamespace;
         public readonly string Name;
 
-        public bool IsGlobalNamespace => ParentNamespace == null;
+        public bool IsGlobalNamespace => ParentNamespace is null;
 
         public override string IdSuffix => ";namespace";
 
@@ -85,8 +85,8 @@ public override IEnumerable<IExtractionProduct> Contents
             get
             {
                 yield return Tuples.namespaces(this, Name);
-                if (!IsGlobalNamespace)
-                    yield return Tuples.parent_namespace(this, ParentNamespace!);
+                if (ParentNamespace is object)
+                    yield return Tuples.parent_namespace(this, ParentNamespace);
             }
         }
     }