fix: handle empty parseResponse for nested script execution (issue #136)

[sonarta]

Problem

When executing a RouterOS script that calls another script (/system/script/run), the response may contain empty data blocks. This causes Undefined array key 0 error at line 377 of Client.php because parseResponse() returns an empty array [].

Solution

• Replace inefficient double parseResponse() call with single call
• Use !empty() and isset() checks to properly handle empty array responses
• Add unit test for empty response edge case

Changes

• src/Client.php: Fixed rosario() method to handle empty parseResponse result
• tests/ClientTest.php: Added test_parseResponse_emptyArrayHandling() test

Testing

• PHP syntax validation passed
• Unit test added for the edge case

Fixes: #136