scripts: treemaps: Fixed crashes when there's nothing to show

Crashing on invalid input isn't the _worst_ behavior, but with a few
tweaks we can make these scripts more-or-less noop in such cases. This
is useful when running with -k/--keep-open since intermediate file
states often contain garbage.

(Ironically one of the precise problems littlefs is trying to solve.)

Also added a special case to treemap.py/codemap.py to not output the
canvas if there's nothing to show and height is implicit. Otherwise the
history mode with -n/--lines ends up filled with blank lines.

Note this makes -H1 subtly different from no -H/--height, with -H1
printing a blank line if there is nothing to show. The -H1 behavior may
also be useful in niche cases where you want that part of the screen
cleared.

---

This was found while trying to run codemap.py -k -n5 during compilation.
GCC writes object files incrementally, and this was breaking our script.

scripts/codemap.py

@@ -1019,6 +1019,8 @@ def main_(f, paths, *,
     # merge code/stack/ctx results
     functions = co.OrderedDict()
     for r in results:
+        if 'function' not in r:
+            continue
         if r['function'] not in functions:
             functions[r['function']] = {'name': r['function']}
         # code things
@@ -1155,6 +1157,10 @@ def main_(f, paths, *,
 
     # assign colors/chars/labels to code tiles
     for i, t in enumerate(code.leaves()):
+        # skip the top tile, yes this can happen if we have no code
+        if t.depth == 0:
+            continue
+
         t.color = subsystems[t.attrs['subsystem']]['color']
 
         if (i, t.attrs['name']) in chars_:
@@ -1218,6 +1224,11 @@ def main_(f, paths, *,
                         ((total_value * to_scale) / (width_*xscale))
                             / yscale)
 
+    # as a special case, if height is implicit and we have nothing to
+    # show, don't print anything
+    if height is None and nil_code and nil_frames and nil_ctx:
+        height_ = 1 if not no_header else 0
+
     # our general purpose partition function
     def partition(tile, **args):
         if tile.depth == 0: