Change denoising_strength default to None.
diff --git a/modules/processing.py b/modules/processing.py
index e124e7f0dd2..061d9955a24 100644
--- a/modules/processing.py
+++ b/modules/processing.py
@@ -142,7 +142,7 @@ class StableDiffusionProcessing:
     overlay_images: list = None
     eta: float = None
     do_not_reload_embeddings: bool = False
-    denoising_strength: float = 0
+    denoising_strength: float = None
     ddim_discretize: str = None
     s_min_uncond: float = None
     s_churn: float = None

Fixes a bug where `Denoising strength: 0` is added to the metadata of API images unless explicitly set to `None` in the API payload, causing hires fix to be always enabled when reading the metadata from an image, even if the image didn't use hires fix to begin with.

## Checklist:

- [x] I have read [contributing wiki page](https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Contributing)
- [x] I have performed a self-review of my own code
- [x] My code follows the [style guidelines](https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Contributing#code-style)
- [x] My code passes [tests](https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Tests)
