Avoid warning about invalid OpenAPI version

Do not emit warning about an invalid OpenAPI version in the `dotnet-getdocument` tool if no explicit value was passed to the tool via the `` argument.
Relates to dotnet#57044.

src/Tools/GetDocumentInsider/src/Commands/GetDocumentCommandWorker.cs

@@ -326,7 +326,10 @@ private string GetDocument(
                 }
                 else
                 {
-                    _reporter.WriteWarning(Resources.FormatInvalidOpenApiVersion(_context.OpenApiVersion));
+                    if (!string.IsNullOrWhiteSpace(_context.OpenApiVersion))
+                    {
+                        _reporter.WriteWarning(Resources.FormatInvalidOpenApiVersion(_context.OpenApiVersion));
+                    }
                     arguments = [documentName, writer, OpenApiSpecVersion.OpenApi3_0];
                 }
             }