fix(PackConstants): don't start new storage buffer if current one is still empty (#23927)

The PackConstants pass introduces a new buffer when a constant cannot be
packed into the current buffer without exceeding the maximum size.
However, if a constant is itself larger than the limit, it gets still
added, but in a separate buffer.
This change fixes a bug in the packing logic when a constant bigger than
the limit is encountered when the current buffer is still empty. In this
case, the current buffer stayed empty and the big constant got added to
a new buffer. The empty buffer caused errors of size 0 buffers not being
supported in following passes or in the runtime. This change avoids
adding a new buffer is the current one is empty and thus avoids these
errors.

Signed-off-by: Stefan Schuermans <schuermans@roofline.ai>