gs-plugin-appstream: Ignore refine for apps with no ID

It can happen a plugin (in my case PackageKit) returns apps for update, which do not have set their ID. Trying to fins such app in an XbSilo resulted in an error:

   failed to process component/id[text()='(null)']: cannot parse text or number `null`

which prevented the Updates page to show anything. Ignoring such apps does not influence the System Updates, these bare packages are part of it too.

---

The console showed this error message, which was also reflected in the Updates page in a shorter form:

09:21:45:0659 Gs  updates-shell: failed to get updates: failed to process component/id[text()='(null)']:
   cannot parse text or number `null`

There had been two affected packages here:

GsApp: [0x302a510]
kind:                generic
state:               unknown
progress:            unknown
bundle-kind:         package
kudo-percentage:     0
name:                shim-ia32
version:             15.6-1
summary:             First-stage UEFI bootloader
source-00:           shim-ia32
source-id-00:        shim-ia32;15.6-1;x86_64;updates
management-plugin:   packagekit
reviews:             0
provided:            0
size-installed:      unknown
size-installed-dependencies: 0 bytes
size-download:       unknown
size-download-dependencies: 0 bytes
size-cache-data:     unknown
size-user-data:      unknown
{GnomeSoftware::PackagingFormat}: RPM
{GnomeSoftware::Creator}: packagekit


GsApp: [0x302a7a0]
kind:                generic
state:               unknown
progress:            unknown
bundle-kind:         package
kudo-percentage:     0
name:                shim-x64
version:             15.6-1
summary:             First-stage UEFI bootloader
source-00:           shim-x64
source-id-00:        shim-x64;15.6-1;x86_64;updates
management-plugin:   packagekit
reviews:             0
provided:            0
size-installed:      unknown
size-installed-dependencies: 0 bytes
size-download:       unknown
size-download-dependencies: 0 bytes
size-cache-data:     unknown
size-user-data:      unknown
{GnomeSoftware::PackagingFormat}: RPM
{GnomeSoftware::Creator}: packagekit

while I see both of them being part of the System Updates proxy app, only without version numbers.

Note this can be reproduced also on the stable branch.