Revert "Do not return null paint if shaderFP is null"

This reverts commit 0d96175c97.

Reason for revert: skia:7583

Also, LG probably doesn't really care about the original fix
so we should be fine for reverting the CL.

Original change's description:
> Do not return null paint if shaderFP is null
>
> This fixes the ripple bug reported by LG:
> https://b.corp.google.com/issues/68964656
>
> In that bug, the button is so tall that our image shader returns
> a null fragment processor because the height exceeds GL_MAX_TEXTURE_SIZE.
> If we return null paint, the ripple is completely gone. This CL returns
> a default paint so we can still see the ripple, and that matches HWUI's
> behaviour.
>
> A GM will be added later after some other urgent Android ripple bug fixes.
>
> Bug: skia:
> Change-Id: I9bcafc078916a6a15fbd84d2019f39ac88d2b2f8
> Reviewed-on: https://skia-review.googlesource.com/73200
> Commit-Queue: Yuqian Li <liyuqian@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

TBR=egdaniel@google.com,bsalomon@google.com,liyuqian@google.com

Bug: skia:7583
Change-Id: I0c8fe994bf0ae07d9f52eafee79adcfcbd0c6445
Reviewed-on: https://skia-review.googlesource.com/110200
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Yuqian Li <liyuqian@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>

src/gpu/SkGr.cpp

@@ -364,6 +364,9 @@ static inline bool skpaint_to_grpaint_impl(GrContext* context,
             shaderFP = std::move(*shaderProcessor);
         } else if (const auto* shader = as_SB(skPaint.getShader())) {
             shaderFP = shader->asFragmentProcessor(fpArgs);
+            if (!shaderFP) {
+                return false;
+            }
         }
     }