doc: _scripts: gen_devicetree_rest: skip empty vendors

Don't create output sections for vendors without bindings. This only
affects the `Unknown vendor` and `Generic or vendor-independent`
vendors, which are populated by default but do not necessarily contain
any entries.

Signed-off-by: Jordan Yates <jordan@embeint.com>

doc/_scripts/gen_devicetree_rest.py

@@ -322,7 +322,9 @@ def write_bindings_rst(vnd_lookup, out_dir):
     .. rst-class:: rst-columns
     ''', string_io)
 
-    for vnd in vnd_lookup.vnd2bindings:
+    for vnd, bindings in vnd_lookup.vnd2bindings.items():
+        if len(bindings) == 0:
+            continue
         print(f'- :ref:`{vnd_lookup.target(vnd)}`', file=string_io)
 
     print_block('''\
@@ -358,6 +360,9 @@ def write_bindings_rst(vnd_lookup, out_dir):
                 title += f' ({vnd})'
         underline = '=' * len(title)
 
+        if len(bindings) == 0:
+            continue
+
         print_block(f'''\
         .. _{vnd_lookup.target(vnd)}: