xen/dt: of_property_read_string return -ENODATA when !length

When the length of the string is zero of_property_read_string should
return -ENODATA according to the description of the function.

However, of_property_read_string doesn't check prop->length. If
prop->length is zero, return -ENODATA.

Without this patch the following command in u-boot:

fdt set /chosen/node property-name

results in of_property_read_string returning -EILSEQ when attempting to
read property-name. With this patch, it returns -ENODATA as expected.

This commit is a backport of:
https://lore.kernel.org/xen-devel/20220416003028.1315268-1-sstabellini@kernel.org/

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

xen/common/device_tree.c

@@ -198,7 +198,7 @@ int dt_property_read_string(const struct dt_device_node *np,
 
     if ( !pp )
         return -EINVAL;
-    if ( !pp->value )
+    if ( !pp->length )
         return -ENODATA;
     if ( strnlen(pp->value, pp->length) >= pp->length )
         return -EILSEQ;

xen/include/xen/device_tree.h

@@ -451,6 +451,9 @@ static inline bool_t dt_property_read_bool(const struct dt_device_node *np,
  * doest not have value, and -EILSEQ if the string is not
  * null-terminated with the length of the property data.
  *
+ * Note that the empty string "" has length of 1, thus -ENODATA cannot
+ * be interpreted as an empty string.
+ *
  * The out_string pointer is modified only if a valid string can be decoded.
  */
 int dt_property_read_string(const struct dt_device_node *np,