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"How do I go about encouraging new leaf growth on these branches?" Well, you do not say where in CA you are but in colder regions, you could wait until late May and prune them all the way down if they have yet to leaf out. In warmer areas of CA, where leaf out happened a long time ago, you would be safe to prune them now since What? Did someone just say that May is practically over already??? Well, yes, May is almost over. ;o)) The other alternative would be to, very carefully, do a scratch test with your finger nails and see if you see some "green". Or not. These stems did not get enough water or cold protection during winter and died. You can try to minimize their number in future years by trying to keep the pot in a protected location during winter and keeping the soil moister (you can water weekly but if there is a chance of late frost or dips below/near freezing, I would water deeply & protect when the dip goes a lot below freezing or temperatures stay down a long time). My typical problem is temperatures that go way up (50-70s, for a long time) and down (20s), something that can break dormancy early. Then a frost or freezing temps kill the stems because the sap was starting to flow. Sometimes, I bring some of the potted hydrangeas into the garage or into the house (albeit I quickly bring them out again, as the air is too dry, and I only want the additional protection during the time temps are cold and then their stay inside is over).
I sometimes end with stems that have leaves in the bottom but are dry near the top/end. For those, I cut only some of the dead wood as hydrangeas have adventitious flower buds that could act as "back up" flower buds when the ones at the stem end get zapped if only the end of the stem dies off.
If this is a hydrangea serrata or macrophylla, it may develop flowers from brand new stems that the plant developed this Spring. But the only way to know if this is a remontant hydrangea is to wait and see (or to already know from the plant label information). A second flush should occur in the summer months.
If you still have last year's flowers that are still attached to live stems, these can be deadheaded (not pruned) by cutting the little string that connects the old bloom to the stem (so, do not cut the stem).
Otherwise, your plant is not tall to need pruning and "normally", these plants do not need pruning if you have planted in a location where they can attain their size at maturity mentioned in the plant label.