When I set `path` in ServiceConfig or `contextpath` in ProtocolConfig, RestProtocol throws NPE on this line:
https://github.com/apache/incubator-dubbo/blob/master/dubbo-rpc/dubbo-rpc-rest/src/main/java/org/apache/dubbo/rpc/protocol/rest/RestProtocol.java#L93
```java
Class implClass = ApplicationModel.getProviderModel(url.getServiceKey()).getServiceInstance().getClass();
```

<dubbo:service path="test" interface="org.apache.dubbo.samples.rest.api.facade.UserRestService" ref="userRestService"
                   protocol="rest" validation="true"/>

This is because we use `group/interface:version` as the key when putting ProviderModel:
```java
Application.initProviderModel(group/interface:version, providerModel)
```
but use `group/path:version` to get, as showed above.

I think to unify to `path` would solve this problem.

if (CollectionUtils.isEmpty(protocols) && provider != null) {
            setProtocols(provider.getProtocols());
        }
        convertProtocolIdsToProtocols();
    }
    private void convertProtocolIdsToProtocols() {
        if (StringUtils.isEmpty(protocolIds) && CollectionUtils.isEmpty(protocols)) {
            List<String> configedProtocols = new ArrayList<>();
            configedProtocols.addAll(getSubProperties(Environment.getInstance()
                    .getExternalConfigurationMap(), Constants.PROTOCOLS_SUFFIX));
            configedProtocols.addAll(getSubProperties(Environment.getInstance()
                    .getAppExternalConfigurationMap(), Constants.PROTOCOLS_SUFFIX));
            protocolIds = String.join(",", configedProtocols);
        }
        if (StringUtils.isEmpty(protocolIds)) {
            if (CollectionUtils.isEmpty(protocols)) {
               setProtocols(
                       ConfigManager.getInstance().getDefaultProtocols()
                        .filter(CollectionUtils::isNotEmpty)
                        .orElseGet(() -> {
                            ProtocolConfig protocolConfig = new ProtocolConfig();
                            protocolConfig.refresh();
                            return Arrays.asList(protocolConfig);
                        })
               );
            }
        } else {
            String[] arr = Constants.COMMA_SPLIT_PATTERN.split(protocolIds);
            List<ProtocolConfig> tmpProtocols = CollectionUtils.isNotEmpty(protocols) ? protocols : new ArrayList<>();
            Arrays.stream(arr).forEach(id -> {