fix: fix crash when clear ui commands without commands.

[andycall]

Fix an crash reported by one of our primary customers.

Build fingerprint: 'HUAWEI/ALN-AL00/HWALN:12/HUAWEIALN-AL00/104.0.0.165C00:user/release-keys'
#00 0x00000000000db92c /data/app/~~Dn3YtAsInOgqKCZP4g==/Q==/lib/arm64/libwebf.so (BuildId: 0c7f3e4f445ad0da05101c741e1fa30e1e686bec)
                                                                                                                               webf::UICommandBuffer::clear()
                                                                                                                               webf/bridge/foundation/ui_command_buffer.cc:128:46
#01 0x00000000000d5b54 /data/app/~~tAYRj4sInOgqKCZP4g==/4SM56a8_ZSWPTzKpQ==/lib/arm64/libwebf.so (BuildId: 0c7f3e4f445ad0da05101c741e1fa30e1e686bec)
                                                                                                                               webf::SharedUICommand::clear()
                                                                                                                               webf/bridge/foundation/shared_ui_command.cc:57:18
#02 0x00000000000ebdd4 /data/app/~~DAYRj4sInOgqKCZP4g==/K3c4SM56a8_ZSWPTzKpQ==/lib/arm64/libwebf.so (clearUICommandItems+16) (BuildId: 0c7f3e4f445ad0da05101c741e1fa30e1e686bec)
                                                                                                                               clearUICommandItems
                                                                                                                               webf/bridge/webf_bridge.cc:307:48