🐛 Don't fail on empty input

Reported by Dmitry Bogatov by email.

md2gemini/__init__.py

@@ -72,6 +72,9 @@ def md2gemini(
     table_tag: "The default alt text for table blocks."
     """
 
+    if len(markdown) == 0:
+        return ""
+
     # Pre processing
     # Remove frontmatter
     frontmatterExists = False

tests/test_plain.py

@@ -6,6 +6,12 @@ def f(md):
     return normalize(md2gemini(md, plain=True))
 
 
+def test_empty_input():
+    md = ""
+    gem = ""
+    assert f(md) == gem
+
+
 def test_remove_bold():
     md = "Sentence with **bold** in it."
     gem = "Sentence with bold in it."