Projects with empty devicetree files may fail to compile

[gmarull]

Describe the bug

When we have an empty Devicetree, ie,

/dts-v1/;

/ {

};

The node's dep_ordinal is never initialized because the node graph is empty. This ends up with invalid ordinal tokens (-1) in devicetree_generated.h which in turn produce some cryptic compiler errors, see e.g.

error: pasting "dts_ord_" and "-" does not give a valid preprocessing
token
   95 | #define Z_DEVICE_DT_DEV_ID(node_id) _CONCAT(dts_ord_,
      DT_DEP_ORD(node_id))

...

include/zephyr/devicetree.h:2498:41:
note: in expansion of macro 'DT_FOREACH_OKAY_HELPER'
 2498 | #define DT_FOREACH_STATUS_OKAY_NODE(fn)
      DT_FOREACH_OKAY_HELPER(fn)
            |
	    ^~~~~~~~~~~~~~~~~~~~~~
include/zephyr/device.h:1022:1:
note: in expansion of macro 'DT_FOREACH_STATUS_OKAY_NODE'
     1022 |
	  DT_FOREACH_STATUS_OKAY_NODE(Z_MAYBE_DEVICE_DECLARE_INTERNAL)

(devicetree_generated.h)

...
 #define DT_N_ORD -1
 #define DT_N_ORD_STR_SORTABLE 000-1
...

To Reproduce

Delete board.dts file, try to compile and observe the errors shown above.

Expected behavior

Failure is not due to missing internal Devicetree macros.

Impact

Certain DT paths need to be guarded with HAS_DTS. Blocker for #63696

Logs and console output

See above.

Environment (please complete the following information):

• OS: Linux
• Toolchain Zephyr SDK 0.16
• Commit SHA or Version used 4d59868

Additional context
N/A