Fix TrueTypeFontUnicode when metrics contains null

[raulfpl]

TrueTypeFontUnicode WriteFont() method throws when metrics contains null values. This commit fixes the issue by filtering out null values

[what-the-diff]

PR Summary

• Improved Metrics Array Initialization
  This change ensures that the metrics array only includes valid entries. Before, we could potentially have null values in our array. Now, we're filtering those out. This improvement could lead to enhanced performance and fewer errors related to null value handling.