Switch to if (...) else in simplify_vector_argument()

[sharlagelfand]

As per @rolfsimoes' suggestion #180 (comment):

One small suggestion on this line. Since simplify_vector is expected to be a single logical value, it might be clearer and more idiomatic to use a standard if (...) else statement:

if (!is.null(simplify_vector)) simplify_vector else getOption("rstac.simplify_vector", default = TRUE)

This avoids the vectorized behavior of ifelse() and aligns better with typical usage for scalar conditions. Just a readability/clarity suggestion. Feel free to take or leave it.