fix(gatsby-transformer-remark): Add fallback when markdown doesn't include excerpt_separator

[Patouche]

Description

When plugin option excerpt_separator is defined and the markdown content don't have the separator, add fallback to the default truncate method.

Related Issues

Fixes #15286

[sidharthachatterjee]

Looks good! Thank you so much @Patouche 👍

[gatsbot]

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