equal: code doesn't correctly differentiate between absence and zero values in maps

[misberner]

The generated code doesn't check for presence in the map in other. This may lead to spuriously returning true if:

1. both maps are of equal size,
2. keys present in both maps map to the same respective values, and
3. keys present in only the first map map to the zero value for the respective type.

Failing test case:

func TestEqualVT_Map_AbsenceVsZeroValue(t *testing.T) {
	a := &TestAllTypesProto3{
		MapInt32Int32: map[int32]int32{
			1: 0,
			2: 37,
		},
	}
	b := &TestAllTypesProto3{
		MapInt32Int32: map[int32]int32{
			2: 37,
			3: 42,
		},
	}

	aJson, err := protojson.Marshal(a)
	require.NoError(t, err)
	bJson, err := protojson.Marshal(b)
	require.NoError(t, err)

	if a.EqualVT(b) {
		assert.JSONEq(t, string(aJson), string(bJson))
		err := fmt.Errorf("these %T should not be equal:\nmsg = %+v\noriginal = %+v", a, a, b)
		require.NoError(t, err)
	}
}