linter/scala/metals: Fix return value of GetProjectRoot()

It was returning 0 when it should be returning an empty string.

The 'AssertEqual' in the ale image is from an old version so it does not
check the types of the arguments.

This is already fixed in https://github.com/junegunn/vader.vim/commit/427fe19104c15066e4c1d5d385076e8e07a0dee8

Closes #3120

ale_linters/scala/metals.vim

@@ -32,6 +32,8 @@ function! ale_linters#scala#metals#GetProjectRoot(buffer) abort
             \)
         endif
     endfor
+
+    return ''
 endfunction
 
 function! ale_linters#scala#metals#GetCommand(buffer) abort