llama : update llama_timings.n_p_eval setting (#7160)

This commit changes the value assigned to llama_timings.n_p_eval when
ctx->n_p_eval is 0 to be 1 instead of 1 which is the current value.

The motivation for this change is that if session caching is enabled,
for example using the `--prompt-cache main-session.txt` command line
argument for the main example, and if the same prompt is used then on
subsequent runs, the prompt tokens will not actually be passed to
llama_decode, and n_p_eval will not be updated by llama_synchoronize.

But the value of n_p_eval will be set 1 by llama_get_timings because
ctx->n_p_eval will be 0. This could be interpreted as 1 token was
evaluated for the prompt which could be misleading for applications
using this value.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>