The DocBlock for this function says this should return sring or false
but find_file_upward actually returns null when no file is found. This
was causing a problem later on because you can't give Spyc::YAMLLoad() a
null variable.
```
PHP  15. Mustangostang\Spyc::YAMLLoad($input = NULL)
/Users/isla/source/wp-cli-dev/wp-cli/php/commands/src/CLI_Alias_Command.php:358
PHP  16. Mustangostang\Spyc->_load($input = NULL)
/Users/isla/source/wp-cli-dev/spyc/src/Spyc.php:118
PHP  17. Mustangostang\Spyc->loadFromSource($input = NULL)
/Users/isla/source/wp-cli-dev/spyc/src/Spyc.php:440
PHP  18. Mustangostang\Spyc->loadFromString($input = NULL)
/Users/isla/source/wp-cli-dev/spyc/src/Spyc.php:512
PHP  19. explode($separator = '\n', $string = NULL)
/Users/isla/source/wp-cli-dev/spyc/src/Spyc.php:516
Deprecated: explode(): Passing null to parameter #2 ($string) of type
string is deprecated in /Users/isla/source/wp-cli-dev/spyc/src/Spyc.php
on line 516
```

Also fix logging

Need to add an extra debug statement when creating a config file because
the existing debug details get overwritten and it only prints the last
one

To see if this solves current CI problems