Fix #2440 - Fix the kotlinc command when Maven and Gradle are missing

ale_linters/kotlin/kotlinc.vim

@@ -12,6 +12,8 @@ let g:ale_kotlin_kotlinc_module_filename = get(g:, 'ale_kotlin_kotlinc_module_fi
 let s:classpath_sep = has('unix') ? ':' : ';'
 
 function! ale_linters#kotlin#kotlinc#RunWithImportPaths(buffer) abort
+    let l:command = ''
+
     " exec maven/gradle only if classpath is not set
     if ale#Var(a:buffer, 'kotlin_kotlinc_classpath') isnot# ''
         return ale_linters#kotlin#kotlinc#GetCommand(a:buffer, [], {})