Math: document that Quaternion::rotationAxis() might return NaN vector.

src/Math/Quaternion.h

@@ -170,7 +170,9 @@ template<class T> class Quaternion {
         /**
          * @brief Rotation axis of unit quaternion
          *
-         * Expects that the quaternion is normalized. @f[
+         * Expects that the quaternion is normalized. Returns either unit-length
+         * vector for valid rotation quaternion or NaN vector for
+         * default-constructed quaternion. @f[
          *      \boldsymbol a = \frac{\boldsymbol q_V}{\sqrt{1 - q_S^2}}
          * @f]
          * @see rotationAngle(), rotation()

src/Math/Test/QuaternionTest.cpp

@@ -213,6 +213,10 @@ void QuaternionTest::rotation() {
     CORRADE_COMPARE(q2, Quaternion(Vector3(-0.5f, -0.5f, -0.5f), 0.5f));
     CORRADE_COMPARE(q2.rotationAngle(), deg(120.0f));
     CORRADE_COMPARE(q2.rotationAxis(), -axis);
+
+    /* Default-constructed quaternion has zero angle and NaN axis */
+    CORRADE_COMPARE(Quaternion().rotationAngle(), deg(0.0f));
+    CORRADE_VERIFY(Quaternion().rotationAxis() != Quaternion().rotationAxis());
 }
 
 void QuaternionTest::angle() {