fix(pickers): when no lnum given, don't set entry.lnum (#2865)

Fixes couple of regressions from #2791 when no line number info is given
in the prompt (eg. `file.txt` rather than `file.txt:10`).

Both issues stemming from the fact that when no line number info is
given, the `entry.lnum` was set to `0`. `entry.lnum` is 1-index.
- Sending results to quickfix would send faulty results (closes #2864)
- Will no open the file on the correct (previous) line number

For this, setting the lnum to 1 is still undesirable since this
overwrites the lnum priority handling in the `select` action.

lua/tests/automated/utils_spec.lua

@@ -108,8 +108,6 @@ describe("separates file path location", function()
     {
       input = "file:",
       file = "file",
-      row = 0,
-      col = 0,
     },
   }