Fix always-true response.c_str() guards in Device_WiFi_SSID RPC response handling

[Copilot]

std::string::c_str() never returns null, so if (response.c_str()) was always true — silently passing empty RPC responses into cJSON_Parse and producing misleading parse error logs.

Changes

• GetConnectedSSID / GetWifiState response guards — replaced if (response.c_str()) with if (!response.empty()) to match the already-correct guard used for the GetAvailableInterfaces response

// Before — always evaluates true, even for empty response
if (response.c_str()) { ... }

// After — correctly skips processing on empty response
if (!response.empty()) { ... }

All three RPC response checks in get_Device_WiFi_SSID_Fields (non-RDKV_NM path) now consistently use !response.empty().

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