A bare `boot` (no argument, argc == 1) passes argv[1] == NULL.
nsh_getfullpath(NULL) returns strdup(g_home) == "/" instead of NULL,
which turned the default-boot path (NULL -> board default image)
into "/". board_boot_image("/") then failed with -EINVAL (-22),
preventing the board from booting.
Guard the nsh_getfullpath() call so NULL passes through unchanged,
restoring the original default-boot behavior while still resolving
relative paths when an argument is given.
This fixes the regression introduced by the relative-path support
```
commit fabafbc361 (origin/master, origin/HEAD)
Author: Junbo Zheng <zhengjunbo1@xiaomi.com>
Date: Fri Jul 24 23:47:44 2026 +0800
nshlib: add relative image path support in boot command
cmd_boot passed the image path straight to boardctl(), which resolves
it in a context that does not inherit the NSH shell cwd, so relative
paths failed and only absolute paths worked. Use nsh_getfullpath() to
resolve relative paths against the cwd before calling boardctl().
Signed-off-by: Junbo Zheng <zhengjunbo1@xiaomi.com>
```
Signed-off-by: Junbo Zheng <zhengjunbo1@xiaomi.com>
145e64a5d4