Don't assume the default barcode type when handling empty string

Addresses <labelle-org#23 (comment)>

Author: maresb

src/labelle/lib/render_engines/barcode.py

@@ -4,13 +4,23 @@
 from PIL import Image
 
 from labelle.lib.barcode_writer import BarcodeImageWriter
-from labelle.lib.constants import DEFAULT_BARCODE_TYPE
+from labelle.lib.constants import DEFAULT_BARCODE_TYPE, BarcodeType
 from labelle.lib.render_engines.render_context import RenderContext
 from labelle.lib.render_engines.render_engine import (
     RenderEngine,
     RenderEngineException,
 )
 
+if DEFAULT_BARCODE_TYPE != BarcodeType.CODE128:
+    # Ensure that we fail fast if the default barcode type is adjusted
+    # and the code below hasn't been updated.
+    raise RuntimeError(
+        "The conditional below assumes that the default barcode type is CODE128. "
+        "Different barcodes have different quirks, so we should manually test the "
+        "new default to ensure a good user experience in the GUI when the initial "
+        "value is an empty string."
+    )
+
 
 class BarcodeRenderError(RenderEngineException):
     def __init__(self):
@@ -25,7 +35,10 @@ def __init__(self, content: str, barcode_type: str | None):
         self.barcode_type = barcode_type or DEFAULT_BARCODE_TYPE
 
     def render(self, context: RenderContext) -> Image.Image:
-        if self.barcode_type == "code128" and self.content == "":
+        if (
+            self.barcode_type == DEFAULT_BARCODE_TYPE == BarcodeType.CODE128
+            and self.content == ""
+        ):
             # An exception is raised on the empty string. Since this is
             # the default code, we really don't want to trigger a popup
             # in the GUI before the user entered a barcode.