kitty: Guard hex color conversion

Sometimes there is a color code, where one part is missing, e.g.
"#[fg=,bg=terminal]", in which case it would try to parse the empty
string as an integer.
Fix this by explicitly checking for an empty argument.

Author: vimpostor

doc/tpipeline-kitty.txt

@@ -77,14 +77,14 @@ def parse_stl(screen: Screen, draw_data: DrawData, stl: str, draw: bool):
 					screen.cursor.fg = as_rgb(int(draw_data.default_fg))
 				elif f == "fg=bg":
 					screen.cursor.fg = screen.cursor.bg
-				else:
+				elif len(f) > 9:
 					screen.cursor.fg = as_rgb(int(f[4:10], 16))
 			elif f.startswith("bg="):
 				if f == "bg=default" or f == "bg=terminal":
 					screen.cursor.bg = as_rgb(int(draw_data.default_bg))
 				elif f == "bg=fg":
 					screen.cursor.bg = screen.cursor.fg
-				else:
+				elif len(f) > 9:
 					screen.cursor.bg = as_rgb(int(f[4:10], 16))
 
 		pos = next + 1