fix(compare): emit replace when a root container type changes from an empty value

[maximilliangrand]

Problem

compare()/generate() produce an empty or corrupt patch when one document root is an empty container and the other is a container of the opposite kind:

compare([], {a: 1})   // => [{op:'add', path:'/a', value:1}]  — applyPatch throws OPERATION_PATH_ILLEGAL_ARRAY_INDEX
compare({}, [1, 2])   // => two 'add' ops that apply to {} as {0:1,1:2}, not [1,2]
compare([], {})       // => []  — applying it leaves the doc as []

This is the exact case reported in #239 and called out earlier in #31 (compare([], {}) === []). The array/object-substitution fix in #205 emits the corrective replace from inside the loop over the source's existing keys, so it never runs when the source container is empty.

Fix

Handle the array-vs-object mismatch once at the top of _generate (after toJSON): emit a single replace of the whole subtree (plus a matching test op under invertible) and return. This also covers the non-empty case previously handled in the loop, so the now-redundant branch is removed. Because _generate recurses, this also fixes the nested cases in #239 (e.g. compare({a:[]}, {a:{}})).

compare([], {a: 1})   // => [{op:'replace', path:'', value:{a:1}}]  ✓ round-trips

Tests

Added compare specs for empty-array-root→object and empty-object-root→array (both invertible variants), each asserting the emitted patch and a round-trip via applyPatch. They fail on master and pass with this change; the rest of the suite is unchanged.

Fixes #239. Also resolves the case documented in #31 that #205 left open.