5.0.1 不配置 service_instance 不上报数据，配置 service_instance 上报为空

[One2r]

Describe the bug
php：7.4.30
skywalking-php-sdk：5.0.1
skywalking-aop：9.2.0

不配置 skywalking.service_instance 不上报数据，配置 skywalking.service_instance 上报数据中 service_instance 为空。

skywalking ext 配置：

skywalking
Directive => Local Value => Master Value
skywalking.enable => On => On
skywalking.service => php => php
skywalking.service_instance => no value => no value
;skywalking.service_instance => 172.31.1.3 => 172.31.1.3
skywalking.oap_version => 9.0.0 => 9.0.0
skywalking.oap_cross_process_protocol => 3.0 => 3.0
skywalking.oap_authentication => no value => no value
skywalking.grpc_address => skywalking-oap:11800 => skywalking-oap:11800
skywalking.grpc_tls_enable => Off => Off
skywalking.grpc_tls_pem_root_certs => no value => no value
skywalking.grpc_tls_pem_private_key => no value => no value
skywalking.grpc_tls_pem_cert_chain => no value => no value
skywalking.log_level => debug => debug
skywalking.log_path => /tmp/skywalking-php.log => /tmp/skywalking-php.log
skywalking.curl_response_enable => Off => Off
skywalking.error_handler_enable => On => On
skywalking.mq_max_message_length => 20480 => 20480
skywalking.mq_unique => 0 => 0
skywalking.sample_n_per_3_secs => -1 => -1```