fix phpstan: use configured level 0 (#5053)

Use configured g:ale_php_phpstan_level if it is 0 (as a number rather
than string).

0 (number) is considered to be empty, but '0' (string) is not.

ale_linters/php/phpstan.vim

@@ -27,6 +27,10 @@ function! ale_linters#php#phpstan#GetCommand(buffer, version) abort
 
     let l:level =  ale#Var(a:buffer, 'php_phpstan_level')
 
+    if type(l:level) is v:t_number
+        let l:level = string(l:level)
+    endif
+
     if empty(l:level) && empty(ale_linters#php#phpstan#FindConfigFile(a:buffer))
         " if no configuration file is found, then use 4 as a default level
         let l:level = '4'