fix(runtime-dom): fix an update error when the style changed from str…

[hg3w]

When I changed the style from string to object, the properties of the original string were not removed:

<div :style="flag ? `color: red;` : { backgroundColor: 'green' }">

<!-- flag changes from true to false and is rendered as: -->
<div style="color: red; background-color: green;"> 

After this PR, the rendering style displays normally:

<div style="background-color: green;"> 

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[haoqunjiang]

Thanks for the PR! Sorry that we didn't notice this one earlier.
Now we've merged #10373 and this issue has been fixed since v3.4.20.