Fix filled(...) throws exception when attribute holds a non-Stringable object

[stevebauman]

Summary

Fixes a TypeError thrown by DummyData::filled() (and isNotFilled(), notFilled(), anyFilled()) when an attribute holds an object that does not implement __toString — most commonly a DateTime returned from Faker\Generator::dateTimeBetween().

Closes #8

Problem

DummyData::isEmptyString() guarded against bool and array before casting the value to a string, but not against objects. Any object without __toString() triggered:

Error: Object of class DateTime could not be converted to string
  at src/DummyData.php:317
  at src/DummyData.php:157

This surfaced for consumers using HasDummyFactory whose toDummyInstance() called $attributes->filled($key) on a key produced by Faker's date-time helpers.

Change

Added ! is_object($value) to the guard in isEmptyString(). Objects are now treated as filled, which matches the semantic intent of filled() — an object is a real, non-empty value.

return ! is_bool($value)
    && ! is_array($value)
    && ! is_object($value)
    && trim((string) $value) === '';