Update spiffs_api.cpp

[seclorum]

Fixes a bug where un-prefixed files are irretrievable with openDir(""). Described: #1818.

[igrr]

You may need to update the test as well. Old behaviour (openDir("") should return empty list) was checked here: https://github.com/esp8266/Arduino/blob/master/tests/host/fs/test_fs.cpp#L163-L164

[hallard]

Just my 2 cents, why not change the original isSpiffsFilenameValid where is the problem, checking len >0 by adding *name == 0 in the 1st if, like this ?

bool isSpiffsFilenameValid(const char* name)
{
    if (name == nullptr || *name == 0) {
        return false;
    }
    auto len = strlen(name);
    return len > 0 && len < SPIFFS_OBJ_NAME_LEN;
}

original is currently

bool isSpiffsFilenameValid(const char* name)
{
    if (name == nullptr) {
        return false;
    }
    auto len = strlen(name);
    return len > 0 && len < SPIFFS_OBJ_NAME_LEN;
}```

[ghost]

Can one of the admins verify this patch?

[igrr]

Reopened as #2143, with test cases fixed.