fix: compiler errors

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -458,7 +458,13 @@ private static void WriteHostInfoV2(IOpenApiWriter writer, IList<OpenApiServer>?
                     .ToList();
 
                 // schemes
-                writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => w.WriteValue(s));
+                writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => 
+                {
+                    if(!string.IsNullOrEmpty(s) && s is not null)
+                    {
+                        w.WriteValue(s);
+                    }
+                });
             }            
         }
 

src/Microsoft.OpenApi/Models/OpenApiOperation.cs

@@ -337,7 +337,13 @@ public void SerializeAsV2(IOpenApiWriter writer)
                     .Distinct()
                     .ToList();
 
-                writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => w.WriteValue(s));
+                writer.WriteOptionalCollection(OpenApiConstants.Schemes, schemes, (w, s) => 
+                {
+                    if (!string.IsNullOrEmpty(s) && s is not null)
+                    {
+                        w.WriteValue(s);
+                    }
+                });
             }
 
             // deprecated

src/Microsoft.OpenApi/Models/OpenApiReference.cs

@@ -177,7 +177,7 @@ private void SerializeInternal(IOpenApiWriter writer, Action<IOpenApiWriter>? ca
         {
             Utils.CheckArgumentNull(writer);
 
-            if (Type == ReferenceType.Tag)
+            if (Type == ReferenceType.Tag && !string.IsNullOrEmpty(ReferenceV3) && ReferenceV3 is not null)
             {
                 // Write the string value only
                 writer.WriteValue(ReferenceV3);
@@ -203,7 +203,7 @@ public void SerializeAsV2(IOpenApiWriter writer)
         {
             Utils.CheckArgumentNull(writer);
 
-            if (Type == ReferenceType.Tag)
+            if (Type == ReferenceType.Tag && !string.IsNullOrEmpty(ReferenceV2) && ReferenceV2 is not null)
             {
                 // Write the string value only
                 writer.WriteValue(ReferenceV2);

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -424,7 +424,13 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteProperty(OpenApiConstants.MinProperties, MinProperties);
 
             // required
-            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => w.WriteValue(s));
+            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => 
+            {
+                if (!string.IsNullOrEmpty(s) && s is not null)
+                {
+                    w.WriteValue(s);
+                }
+            });
 
             // enum
             writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (nodeWriter, s) => nodeWriter.WriteAny(s));
@@ -662,7 +668,13 @@ private void SerializeAsV2(
             writer.WriteProperty(OpenApiConstants.MinProperties, MinProperties);
 
             // required
-            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => w.WriteValue(s));
+            writer.WriteOptionalCollection(OpenApiConstants.Required, Required, (w, s) => 
+            {
+                if (!string.IsNullOrEmpty(s) && s is not null)
+                {
+                    w.WriteValue(s);
+                }
+            });
 
             // enum
             writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (w, s) => w.WriteAny(s));
@@ -815,7 +827,13 @@ where temporaryType.HasFlag(flag)
                         select flag.ToFirstIdentifier()).ToList();
             if (list.Count > 1)
             {
-                writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) => w.WriteValue(s));
+                writer.WriteOptionalCollection(OpenApiConstants.Type, list, (w, s) => 
+                {
+                    if (!string.IsNullOrEmpty(s) && s is not null)
+                    {
+                        w.WriteValue(s);
+                    }
+                });
             }
             else
             {

src/Microsoft.OpenApi/Models/OpenApiServerVariable.cs

@@ -84,7 +84,13 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             writer.WriteProperty(OpenApiConstants.Description, Description);
 
             // enums
-            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (w, s) => w.WriteValue(s));
+            writer.WriteOptionalCollection(OpenApiConstants.Enum, Enum, (w, s) => 
+            {
+                if (!string.IsNullOrEmpty(s) && s is not null)
+                {
+                    w.WriteValue(s);
+                }
+            });
 
             // specification extensions
             writer.WriteExtensions(Extensions, version);