Fix incorrect space before colon in if/while stmts (#1655)

* Fix incorrect space before colon in if/while stmts

Previously Black would format this code

```
if (foo := True):
	print(foo)
```

as

```
if (foo := True) :
	print(foo)
```

adding an incorrect space after the RPAR. Buggy code in the
normalize_invisible_parens function caused the colon to be wrapped in
invisible parentheses. The LPAR of that pair was then prefixed with a
single space at the request of the whitespace function.

This commit fixes the accidental skipping of a pre-condition check
which must return True before parenthesis normalization of a specific
child Leaf or Node can happen. The pre-condition check being skipped
was why the colon was wrapped in invisible parentheses.

* Add an entry in CHANGES.md