power: supply: cpcap-charger: Fix null check for power_supply_get_by_name

power: supply: cpcap-charger: Fix null check for power_supply_get_by_name

In the cpcap_usb_detect() function, the power_supply_get_by_name()
function may return `NULL` instead of an error pointer.
To prevent potential null pointer dereferences, Added a null check.

Fixes: eab4e6d953c1 ("power: supply: cpcap-charger: get the battery inserted infomation from cpcap-battery")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Link: https://lore.kernel.org/r/20250519024741.5846-1-hanchunchao@inspur.com
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>