nshlib: guard nsh_getfullpath against NULL argv[1] in boot command

[Junbo-Zheng]

Summary

A bare boot (no argument, argc == 1) passes argv[1] == NULL. nsh_getfullpath(NULL) returns strdup(g_home) == "/"

nuttx-apps/nshlib/nsh_envcmds.c

Lines 191 to 203 in 6e61ce7

	FAR char *nsh_getfullpath(FAR struct nsh_vtbl_s *vtbl,
	FAR const char *relpath)
	{
	FAR const char *wd;
	/* Handle some special cases */
	if (!relpath || relpath[0] == '\0')
	{
	/* No relative path provided */
	return 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 fabafbc3615b3d22636bbf33f932578f491bef23 (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>

Impact

• Users: Fixes a regression where bare boot (no image argument) stopped working after the relative-path support boot now works again
• Build: None -- one-line source change, no con change.
• Hardware: None -- no board-specific logic touched; affects all boards using NSH boot.
• Documentation: None.
• Security & Compatibility: None

Testing

Passed internal CT tests by running boot with no argument

• before, it returned -EINVAL and no boot occurred
• after, the default boot path proceeds

Signed-off-by: Junbo Zheng zhengjunbo1@xiaomi.com