This exact thing happened to me. I found the answer here on this forum but I can't remember where to link to it so i'll just post the answer myself.
Your content and manifest files aren't json files, they are text files with .json in their name. You can see they are listed as "text document" in the TYPE column. This should say JSON instead. You can fix this by opening up your text files in notepad++, choosing Save As, there is a drop down menu below the file name and you can choose JSON from that list. This saves the file as a new file in JSON format.
You get the error that your assets folder is empty because SMAPI doesn't find any json files in your mod folder, so it looks in your assets folder next. When if finds nothing their either it prints the error.
