Fix empty struct and add test ##types

[ghost]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the documentation and the radare2 book with the relevant information (if needed)

Detailed description
Move up the struct definition before the while loop so an empty struct could be defined.

Test plan

$ r2 -
[0x00000000]> "td struct Empty{};"
[0x00000000]> ts
Empty
[0x00000000]> "td struct Empty{int a;};"
[0x00000000]> ts
Empty

Closing issues
#16342

[radare]

Why it's WIP? looks good to me