Cannot define `isempty` in Main

I found a confusing behavior when I tried to define a function named `isempty` in the `Main` module:
```
julia> isempty(x) = true
ERROR: error in method definition: function Base.isempty must be explicitly imported to be extended
Stacktrace:
 [1] top-level scope
   @ none:0
 [2] top-level scope
   @ REPL[1]:1
```