vimscript: Fix exception on literal empty list

Issue #190

vimscript/step3_env.vim

@@ -43,6 +43,9 @@ function EVAL(ast, env)
   if !ListQ(a:ast)
     return EvalAst(a:ast, a:env)
   end
+  if EmptyQ(a:ast)
+    return a:ast
+  endif
 
   let first_symbol = ObjValue(ObjValue(a:ast)[0])
   if first_symbol == "def!"

vimscript/step4_if_fn_do.vim

@@ -44,6 +44,9 @@ function EVAL(ast, env)
   if !ListQ(a:ast)
     return EvalAst(a:ast, a:env)
   end
+  if EmptyQ(a:ast)
+    return a:ast
+  endif
 
   let first = ListFirst(a:ast)
   let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/step5_tco.vim

@@ -48,6 +48,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/step6_file.vim

@@ -48,6 +48,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/step7_quote.vim

@@ -66,6 +66,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/step8_macros.vim

@@ -96,6 +96,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/step9_try.vim

@@ -110,6 +110,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""

vimscript/stepA_mal.vim

@@ -110,6 +110,9 @@ function EVAL(ast, env)
     if !ListQ(ast)
       return EvalAst(ast, env)
     end
+    if EmptyQ(ast)
+      return ast
+    endif
 
     let first = ListFirst(ast)
     let first_symbol = SymbolQ(first) ? ObjValue(first) : ""