FIX Format string defaults correctly

Before this patch, string defaults are formatted without quotes around them.
However, if the default is a tuple or list of strings, quotes will be included:

```
default="a" ==> :default: ``a``
default=("a",) ==> :default: ``('a')``
```

This is a slightly annoying inconsistency, and leads to potential for confusion
between `default="0"` and `default=0`. Most bothersome is that in the case of an
empty string we get:

```
:default: ````
```

Which makes docutils angry:

```
CRITICAL: Unexpected section title or transition.

````
```

This fixes the trouble by formatting the repr of the default.

Author: hoodmane

releasenotes/notes/better-string-defaults-3664ae102b044972.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Now when the default value of an option is a string it will be rendered with
+    quotes around it.
+fixes:
+  - |
+    If an option has an empty string default, docutils will no longer emit
+    warnings saying `CRITICAL: Unexpected section title or transition.`

sphinx_click/ext.py

@@ -104,18 +104,18 @@ def _write_opts(opts: ty.List[str]) -> str:
     else:
         show_default = ctx.show_default
 
-    if isinstance(show_default, str):
+    if show_default and isinstance(show_default, str):
         # Starting from Click 7.0 show_default can be a string. This is
         # mostly useful when the default is not a constant and
         # documentation thus needs a manually written string.
         extras.append(':default: ``%s``' % show_default)
-    elif opt.default is not None and show_default:
+    elif show_default and opt.default is not None:
         extras.append(
             ':default: ``%s``'
             % (
                 ', '.join(str(d) for d in opt.default)
                 if isinstance(opt.default, (list, tuple))
-                else opt.default,
+                else repr(opt.default),
             )
         )
 

tests/test_formatter.py

@@ -243,6 +243,8 @@ def test_defaults(self):
             '--only-show-default',
             show_default="Some default computed at runtime!",
         )
+        @click.option('--string-default', default="abc", show_default=True)
+        @click.option('--empty-string-default', default="", show_default=True)
         def foobar(bar):
             """A sample command."""
             pass
@@ -277,6 +279,14 @@ def foobar(bar):
         .. option:: --only-show-default <only_show_default>
 
             :default: ``Some default computed at runtime!``
+
+        .. option:: --string-default <string_default>
+
+            :default: ``'abc'``
+
+        .. option:: --empty-string-default <empty_string_default>
+
+            :default: ``''``
         """
             ).lstrip(),
             '\n'.join(output),