Do not apply `style_semicolon` on empty statements

[colluca]

Currently, applying textrules.style_semicolon to the following code generates an error on the default line:

unique case (csr_addr.address)
  riscv_instr::CSR_MSTATUS: begin
    // Some statements
  end
  default: ;
endcase

However, the code is arguably properly-styled, and this is IMO rather an exception that would be best handled by excluding it from textrules.style_semicolon.

What do you think about this?