fix(api): nvim_buf_get_offset in a new buffer with zero or one lines

fixes #24930

src/nvim/memline.c

@@ -3934,6 +3934,11 @@ long ml_find_line_or_offset(buf_T *buf, linenr_T lnum, long *offp, bool no_ff)
   if (buf->b_ml.ml_usedchunks == -1
       || buf->b_ml.ml_chunksize == NULL
       || lnum < 0) {
+    // memline is currently empty. Although if it is loaded,
+    // it behaves like there is one empty line.
+    if (!ffdos && buf->b_ml.ml_mfp && (lnum == 1 || lnum == 2)) {
+      return lnum - 1;
+    }
     return -1;
   }
 

test/functional/api/buffer_spec.lua

@@ -1151,6 +1151,18 @@ describe('api/buf', function()
       eq(6, bufmeths.get_offset(1,1))
       command("bunload! 1")
       eq(-1, bufmeths.get_offset(1,1))
+      eq(-1, bufmeths.get_offset(1,0))
+    end)
+
+    it('works in empty buffer', function()
+      eq(0, get_offset(0))
+      eq(1, get_offset(1))
+    end)
+
+    it('works in buffer with one line inserted', function()
+      feed('itext')
+      eq(0, get_offset(0))
+      eq(5, get_offset(1))
     end)
   end)