Fix zero tangent guard in polar pullback

Guard `C .+= ΔP` with `!iszerotangent(ΔP)` in both
`left_polar_pullback!` and `right_polar_pullback!` to handle
the case where ΔP is `nothing`.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lkdvos

src/pullbacks/polar.jl

@@ -17,7 +17,7 @@ function left_polar_pullback!(ΔA::AbstractMatrix, A, WP, ΔWP; kwargs...)
     !iszerotangent(ΔW) && mul!(M, W', ΔW, 1, 1)
     !iszerotangent(ΔP) && mul!(M, ΔP, P, -1, 1)
     C = _sylvester(P, P, M' - M)
-    C .+= ΔP
+    !iszerotangent(ΔP) && (C .+= ΔP)
     ΔA = mul!(ΔA, W, C, 1, 1)
     if !iszerotangent(ΔW)
         ΔWP = ΔW / P
@@ -47,7 +47,7 @@ function right_polar_pullback!(ΔA::AbstractMatrix, A, PWᴴ, ΔPWᴴ; kwargs...
     !iszerotangent(ΔWᴴ) && mul!(M, ΔWᴴ, Wᴴ', 1, 1)
     !iszerotangent(ΔP) && mul!(M, P, ΔP, -1, 1)
     C = _sylvester(P, P, M' - M)
-    C .+= ΔP
+    !iszerotangent(ΔP) && (C .+= ΔP)
     ΔA = mul!(ΔA, C, Wᴴ, 1, 1)
     if !iszerotangent(ΔWᴴ)
         PΔWᴴ = P \ ΔWᴴ