fix(api): make nvim_get_hl not return non-existing groups

fixes #23063

src/nvim/highlight_group.c

@@ -1530,6 +1530,10 @@ static bool hlgroup2dict(Dictionary *hl, NS ns_id, int hl_id, Arena *arena)
   if (link == -1) {
     return false;
   }
+  if (ns_id == 0 && sgp->sg_cleared && sgp->sg_set == 0) {
+    // table entry was created but not ever set
+    return false;
+  }
   HlAttrs attr =
     syn_attr2entry(ns_id == 0 ? sgp->sg_attr : ns_get_hl(&ns_id, hl_id, false, sgp->sg_set));
   *hl = arena_dict(arena, HLATTRS_DICT_SIZE + 1);

test/functional/api/highlight_spec.lua

@@ -575,4 +575,25 @@ describe('API: get highlight', function()
     meths.set_hl(0, 'Foo', hl)
     eq(hl, meths.get_hl(0, { name = 'Foo', link = true }))
   end)
+
+  it("doesn't contain unset groups", function()
+    local id = meths.get_hl_id_by_name "@foobar.hubbabubba"
+    ok(id > 0)
+
+    local data = meths.get_hl(0, {})
+    eq(nil, data["@foobar.hubbabubba"])
+    eq(nil, data["@foobar"])
+
+    command 'hi @foobar.hubbabubba gui=bold'
+    data = meths.get_hl(0, {})
+    eq({bold = true}, data["@foobar.hubbabubba"])
+    eq(nil, data["@foobar"])
+
+    -- @foobar.hubbabubba was explicitly cleared and thus shows up
+    -- but @foobar was never touched, and thus doesn't
+    command 'hi clear @foobar.hubbabubba'
+    data = meths.get_hl(0, {})
+    eq({}, data["@foobar.hubbabubba"])
+    eq(nil, data["@foobar"])
+  end)
 end)