Given: \[\begin{split} Q &: {\varnothing}= {{\mathbf{\top}}}\\ R &: 0 \leq y {\ \land \ }(0 \leq x \to y = x) {\ \land \ }(x < 0 \to y = -x) \end{split}\]
Then: \[\begin{split} & {wp\left({{if\left(x < 0\right) \ \{ y := -x \}} \ {else \ \{ y := x \}}}, R\right)} \\ {\ ({\mathbf{§4}})} \iff & \left(x < 0 \to {wp\left(y:= -x, R\right)}\right) {\ \land \ }\left(0 \leq x \to {wp\left(y:= x, R\right)}\right)\\ {\ ({\mathbf{§1}})} \iff & (x < 0 \to (0 \leq -x {\ \land \ }(0 \leq x \to -x = x) {\ \land \ }(x < 0 \to -x = -x )))\\ \land & (x \leq 0 \to (0 \leq x {\ \land \ }(0 \leq x \to x = x) {\ \land \ }(x < 0 \to x = -x)))\\ \iff & (x < 0 \to -x = -x) {\ \land \ }(0 \leq x \to x = x)\\ \iff & {{\mathbf{\top}}}\\ \end{split}\]
Therefore: \[\begin{split}Q \implies& {{\mathbf{\top}}}\end{split}\]
In other words, the postcondition holds for all possible preconditions.