nacos registry serviceName may conflict

[uglycow]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: xxx
• Operating System version: xxx
• Java version: xxx

Steps to reproduce this issue

nacos registry serviceName may conflict when we have two provider with same service but one config version as A and left group empty and
one config group as A and left version empty

the two provider would get registered with same key provdier:serviceName:A.

the cause of this issue is we ignore empty value when we construct the key, relative code is

 private String getServiceName(URL url) {
        String category = url.getParameter(CATEGORY_KEY, DEFAULT_CATEGORY);
        return getServiceName(url, category);
    }

    private String getServiceName(URL url, String category) {
        StringBuilder serviceNameBuilder = new StringBuilder(category);
        append(serviceNameBuilder, url, INTERFACE_KEY);
        append(serviceNameBuilder, url, VERSION_KEY);
        append(serviceNameBuilder, url, GROUP_KEY);
        return serviceNameBuilder.toString();
    }

private void append(StringBuilder target, URL url, String parameterName) {
        target.append(SERVICE_NAME_SEPARATOR);
        String parameterValue = url.getParameter(parameterName);
        if (!StringUtils.isBlank(parameterValue)) {
            target.append(parameterValue);
        }
    }

i think we should just keep a SEPARATOR as placeholder for empty value.

1. xxx
2. xxx
3. xxx

Pls. provide [GitHub address] to reproduce this issue.

Expected Result

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Actual Result

What actually happens?

If there is an exception, please attach the exception trace:

Just put your stack trace here!

[ralf0131]

This should be fixe from 2.7.2 onwards.