Correct clamp annotations for callable formatters and None (#516)

Co-authored-by: Chandra Tekulapally <chandralohit1234@gmail.com>

Author: hugovk

src/humanize/number.py

@@ -15,6 +15,7 @@
 
 TYPE_CHECKING = False
 if TYPE_CHECKING:
+    from collections.abc import Callable
     from typing import TypeAlias
 
     # This type can be better defined by typing.SupportsFloat
@@ -458,13 +459,13 @@ def scientific(value: NumberOrString, precision: int = 2) -> str:
 
 
 def clamp(
-    value: float,
-    format: str = "{:}",
+    value: float | None,
+    format: str | Callable[[float], str] = "{:}",
     floor: float | None = None,
     ceil: float | None = None,
     floor_token: str = "<",
     ceil_token: str = ">",
-) -> str:
+) -> str | None:
     """Returns number with the specified format, clamped between floor and ceil.
 
     If the number is larger than ceil or smaller than floor, then the respective limit
@@ -488,7 +489,7 @@ def clamp(
         ```
 
     Args:
-        value (int, float): Input number.
+        value (int, float, None): Input number, or `None` to return `None`.
         format (str OR callable): Can either be a formatting string, or a callable
             function that receives value and returns a string.
         floor (int, float): Smallest value before clamping.
@@ -499,7 +500,8 @@ def clamp(
             to output.
 
     Returns:
-        str: Formatted number. The output is clamped between the indicated floor and
+        str | None: Formatted number, or `None` when `value` is `None`.
+            The output is clamped between the indicated floor and
             ceil. If the number is larger than ceil or smaller than floor, the output
             will be prepended with a token indicating as such.