remove one test of ismissing, that is no longer possible and set a changelog date.

Author: kellertuer

Changelog.md

@@ -6,7 +6,7 @@ The file was started with Version `0.4`.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [0.6.0] unreleased
+## [0.6.0] June 24, 2026
 
 This is a breaking change since the JuMP extension is dropped.
 We also unified a few of the internal solver state constructors.

src/plans/nonlinear_least_squares/nls_general_plan.jl

@@ -315,7 +315,7 @@ function get_LevenbergMarquardt_scaling(
         threshold::Real = 1.0e-5, mode::Symbol = :Strict
     )
     # second derivative existent and negative: In strict mode (motivated by ceres) -> return sqrt(ρ_prime), 0
-    (ismissing(ρ_double_prime) || (ρ_double_prime < 0 && mode == :Strict)) && return (sqrt(ρ_prime), 0.0)
+    ((ρ_double_prime < 0 && mode == :Strict)) && return (sqrt(ρ_prime), 0.0)
     (iszero(FkSq) && mode == :Strict) && return (sqrt(ρ_prime), 0.0)
     α = 1 - sqrt(max(1 + 2 * (ρ_double_prime / ρ_prime) * FkSq, 0.0))
     α = min(α, 1 - threshold)