Guard vdata_make() against strdup(NULL) for DATA_FORMAT

[1technophile]

A decoder passing a NULL format to data_make/data_int/data_dbl reaches vdata_make()'s DATA_FORMAT case, which calls strdup() on the argument unconditionally. strdup(NULL) is undefined and dereferences a null pointer, crashing the decoder task with a LoadProhibited exception (EXCVADDR 0x0) and rebooting the gateway.

This is reachable from normal operation: digitech_xc0324_decode() emits its 'message_num' field with a NULL format
(data_int(data, "message_num", "Message repeat count", NULL, events)), so any successful XC-0324 decode -- including the false-positive decodes that ambient RF noise produces every few hours -- crashes the device. Captured on an ESP32 as:

Guru Meditation Error: Core 1 panic'ed (LoadProhibited)
... strlen <- _strdup_r <- strdup <- vdata_make <- data_int
<- digitech_xc0324_decode <- pulse_slicer_ppm <- run_ook_demods

Read the format into a temporary and only strdup() it when non-NULL, leaving format NULL otherwise. A NULL format is legitimate and widely used; the output backends already treat it as optional (the JSON string formatter does UNUSED(format)).

Fixes the reboots reported in 1technophile/OpenMQTTGateway#2317.

[NorthernMan54]

@1technophile Caught me at the desktop, published as 0.5.1

Tks very much