get_buffer_lines: Return a string, when requested, on invalid input

Closes #7859

src/nvim/eval.c

@@ -9259,7 +9259,9 @@ static void get_buffer_lines(buf_T *buf, linenr_T start, linenr_T end, int retli
   rettv->vval.v_string = NULL;
 
   if (buf == NULL || buf->b_ml.ml_mfp == NULL || start < 0 || end < start) {
-    tv_list_alloc_ret(rettv, 0);
+    if (retlist) {
+      tv_list_alloc_ret(rettv, 0);
+    }
     return;
   }
 

test/functional/eval/getline_spec.lua

@@ -0,0 +1,39 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local call = helpers.call
+local clear = helpers.clear
+local eq = helpers.eq
+local expect = helpers.expect
+
+describe('getline', function()
+  before_each(function()
+    clear()
+    call('setline', 1, {'a', 'b', 'c'})
+    expect([[
+      a
+      b
+      c]])
+  end)
+
+  it('returns empty string for invalid line', function()
+    eq('', call('getline', -1))
+    eq('', call('getline', 0))
+    eq('', call('getline', 4))
+  end)
+
+  it('returns empty list for invalid range', function()
+    eq({}, call('getline', 2, 1))
+    eq({}, call('getline', -1, 1))
+    eq({}, call('getline', 4, 4))
+  end)
+
+  it('returns value of valid line', function()
+    eq('b', call('getline', 2))
+    eq('a', call('getline', '.'))
+  end)
+
+  it('returns value of valid range', function()
+    eq({'a', 'b'}, call('getline', 1, 2))
+    eq({'a', 'b', 'c'}, call('getline', 1, 4))
+  end)
+end)