I'm building a web application and I need to let the user know when the page is empty because of a lack of data.
What's the correct way to say that?
There are no data...
There is no data...
Is it correct to say
No data available
i.e. omitting the "There is/There are" part?
It's not really a matter of “correctness” whether you say data is or data are. You just need to write what people are expecting to read in this context. But without further information about the context in which it will be used, this is difficult to predict.
As for "No data available", that's perfectly find as a terse computer message or if written in "headlinese" as a section header for some article. It would be difficult to argue that this is not a sentence with a subject and a predicate, despite having no overt finite verb. There’s still a predicate, after all.
But that does not matter for headlinese, so it would be just fine there.
