The C# compiler emits a NOP instruction at a curly brace. Which makes it a lot easier to set breakpoints in your code. The debugger only permits setting a breakpoint on code and a curly brace doesn't normally produce any code. So this is just a simple debugging aid, these NOPs won't get generated in the release build.
The BR.S instruction is a minor flaw in the compiler, it doesn't have a peephole optimizer to get rid of these kind of extraneous instructions. In general, it is not the job of the C# compiler to optimize code, that's done by the jitter. Which will readily and easily remove the instruction.