It's invalid to have an <inlineequation> outside a paragraph. Another solution would have been to change InsetMathHull::docbook to generate an <informalequation>, but that function would have required more knowledge about its context than now.
Need more tests to determine if <inlineequation> should only be output for hullSimple or in more cases like:
const static std::set<HullType> inlineHulls = {
hullUnknown,
hullNone,
hullSimple,
hullEquation,
hullMultline,
hullGather,
hullRegexp
};
