Only try to set flag_value if is_flag is true

This statement:

    flag_value = not self.default

requires the class to have a working `__bool__` method. However,
there are some classes that explicitly disable this method:

- [`numpy.ndarray`](https://numpy.org/doc/stable/reference/arrays.ndarray.html)
  raises `ValueError: The truth value of an array with more than
  one element is ambiguous. Use a.any() or a.all()`
- [`astropy.units.Quantity`](https://docs.astropy.org/en/stable/api/astropy.units.Quantity.html)
  raises `ValueError: Quantity truthiness is ambiguous, especially
  for logarithmic units and temperatures. Use explicit  comparisons.`

Move this statement so that it is only executed if `is_flag` is
true.

src/click/core.py

@@ -2597,11 +2597,10 @@ def __init__(
             else:
                 self.default = False
 
-        if flag_value is None:
-            flag_value = not self.default
-
         self.type: types.ParamType
         if is_flag and type is None:
+            if flag_value is None:
+                flag_value = not self.default
             # Re-guess the type from the flag value instead of the
             # default.
             self.type = types.convert_type(None, flag_value)

tests/test_info_dict.py

@@ -58,7 +58,7 @@
         "help": None,
         "prompt": None,
         "is_flag": False,
-        "flag_value": False,
+        "flag_value": None,
         "count": False,
         "hidden": False,
     },