If you are using Power Automate with SharePoint Online list or library data and need to check if a field is empty or null, then you can use the @empty function.
Example
First, add a Condition and add the field you want to check.
Next, click on the "Edit in advanced mode" link.
Next, you will replace the content in the box with the empty() function.
The "triggerbody()', in this case, is what triggered the flow. This may be different depending on where in your Flow your field is coming from.
The "Title" is the name of the field.
The function returns true if the field is empty.