Fix WebGPU empty tensor identity values: use division-by-zero for infinity
Replace float literal approximations (-3.4028e+38f) with proper
infinity via division-by-zero: output_value_t(-1.0) / output_value_t(0.0)
for -inf, output_value_t(1.0) / output_value_t(0.0) for +inf.
The float literal approach produced -FLT_MAX instead of -inf, causing
test failures where the expected output is -inf (ReduceMax/Min/LogSum
empty tensor tests).
Division by zero is well-defined in IEEE 754 and WGSL (produces ±inf).
Using output_value_t() ensures the expression works for both f32 and
f16 output types.
Signed-off-by: Justin Chu <justinchu@microsoft.com>