fix(compiler-sfc): should keep template nodes with no content (close #2463)

[meteorlxy]

See #2463

This tweak is based on the following consideration:

An empty <template> block means it should render empty string, which is different from no <template> block.

[znck]

SFC descriptor should keep all the blocks as some dev toolings depend on the structure of the file. e.g. VueDX/TS-Plugin auto imports components in <script> block but if descriptor.script is null it adds a <script> block.