selftests/net: initialize char variable to null

selftests/net: initialize char variable to null

char variable in 'so_txtime.c' & 'txtimestamp.c' were left uninitilized
when switch default case taken. which raises following warning.

	txtimestamp.c:240:2: warning: variable 'tsname' is used uninitialized
	whenever switch default is taken [-Wsometimes-uninitialized]

	so_txtime.c:210:3: warning: variable 'reason' is used uninitialized
	whenever switch default is taken [-Wsometimes-uninitialized]

initializing these variables to NULL to fix this.

Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Link: https://patch.msgid.link/20251125165302.20079-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>