modem-helpers: fix parsing CPMS=? responses without groups

The CPMS test parser was expecting 3 groups (parenthesis enclosed lists) of
memory IDs, e.g.:

   +CPMS: ("SM","ME"),("SM","ME"),("SM","ME")

But some modems like the Huawei MU609 may just report single elements, not
groups, e.g.:

   +CPMS: "SM","SM","SM"

This patch avoids using g_strsplit() to split the groups, as that is unaware
of the possible replies without groups. Instead, a new helper method is
implemented which does the group/item split itself, considering also the
possibility of a reply with mixed groups and non-groups, like e.g.:

   +CPMS: ("SM","ME"),"SM","SM"

Additionally, we also now support the case where the groups are empty, e.g.:

   +CPMS: (),(),()

https://bugs.freedesktop.org/show_bug.cgi?id=92243

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