glob2regpat(): handle empty string.

src/nvim/eval.c

@@ -10792,7 +10792,7 @@ static void f_glob2regpat(typval_T *argvars, typval_T *rettv)
   char_u *pat = get_tv_string_chk(&argvars[0]);  // NULL on type error
 
   rettv->v_type = VAR_STRING;
-  rettv->vval.v_string = (pat == NULL)
+  rettv->vval.v_string = (pat == NULL || *pat == NUL)
     ? NULL
     : file_pat_to_reg_pat(pat, NULL, NULL, false);
 }

src/nvim/fileio.c

@@ -7106,6 +7106,7 @@ char_u * file_pat_to_reg_pat(
     char *allow_dirs,        // Result passed back out in here
     int no_bslash            // Don't use a backward slash as pathsep
 )
+  FUNC_ATTR_NONNULL_ARG(1)
 {
   const char_u *endp;
   char_u      *reg_pat;

test/functional/legacy/glob2regpat_spec.lua

@@ -12,6 +12,9 @@ describe('glob2regpat()', function()
     helpers.feed('<cr>')
     neq(nil, string.find(eval('v:errmsg'), '^E806:'))
   end)
+  it('returns empty string for empty input', function()
+    eq('', eval("glob2regpat('')"))
+  end)
   it('handles valid input', function()
     eq('^foo\\.', eval("glob2regpat('foo.*')"))
     eq('\\.vim$', eval("glob2regpat('*.vim')"))