fix SIGSEGV when store config was found but isn't readable

[SKevo18]

No description provided.

[SKevo18]

would break tests otherwise:

could not parse fixture//magento2/configs/empty.php, missing user or db name
open /do/not/exist: no such file or directory
--- FAIL: TestDiscoverStores (0.05s)
    store_test.go:24: 
                Error Trace:    store_test.go:24
                Error:          "[%!s(*gocommerce.Store=&{fixture/discovery/applications/123456789/public_html 0x1016eec60 0x1d8fb903abe8})]" should have 2 item(s), but has 1
                Test:           TestDiscoverStores
panic: runtime error: index out of range [1] with length 1 [recovered, repanicked]

the configs should never be empty anyways(?), so I added a minimal placeholder