logging: Fix easylogging++ init with blank config

[iamamyth]

The upstream version of el::base::TypedConfigurations::unsafeGetConfigByRef accesses uninitialized memory if a key doesn't exist. Commit b2c59af patched the library to throw in this case, avoiding the invalid access, but the more suitable pattern, both logically (configure with a default-initialized config shouldn't throw), and as evidenced by the behavior of unsafeGetConfigByVal, would be to return a const reference to a default-initialized value with static storage duration.

[iamamyth]

For the avoidance of doubt: The library seems quite clearly to intend the "blank config" case to work. For example, in unsafeValidateFileRolling, it checks:

  base::type::fstream_t* fs = unsafeGetConfigByRef(level, &m_fileStreamMap, "fileStream").get();
  if (fs == nullptr) {
    return true;
  }

The structure and behavior of unsafeGetConfigByVal (which logs an error for a missing key and returns a default-initialized object) exactly matching unsafeGetConfigByRef (with the exception of the missing return) evidences a fairly clear intent.