How to apply the pumping lemma for CFGs to languages that contain the empty string
The pumping lemma requires the word $w$ you start with to be large enough &mdash; larger than the pumping lemma constant $p$.

However, this doesn't really answer your question, which is how to ensure that the string given to the pumping lemma is not empty. Here you are very lucky &mdash; you are the one choosing the string! The pumping lemma only states that whatever string $w$ you choose from the language (as long as it is long enough), it can be decomposed as $w = uvxyz$ so that $uv^ixy^iz$ is in the language for all $i \geq 0$, $vy \neq \epsilon$, and $|vxy| \leq p$.