Fix edge-case crash in InlineProcessor

[oprypin]

If an inlineprocessor returns an AtomicString (even though that is pointless, a plain string is atomic in that context), there can be an exception in 2 separate places. The added test case was crashing before this change.

  File "tests/test_apis.py", line 722, in testInlineProcessorDoesntCrashWithAtomicString
    new = self.inlineprocessor.run(tree)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "markdown/treeprocessors.py", line 384, in run
    lst = self.__processPlaceholders(
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "markdown/treeprocessors.py", line 223, in __processPlaceholders
    if child.tail:
       ^^^^^^^^^^
AttributeError: 'AtomicString' object has no attribute 'tail'

  File "tests/test_apis.py", line 722, in testInlineProcessorDoesntCrashWithAtomicString
    new = self.inlineprocessor.run(tree)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "markdown/treeprocessors.py", line 387, in run
    self.__handleInline(text), child
    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "markdown/treeprocessors.py", line 136, in __handleInline
    data, matched, startIndex = self.__applyPattern(
                                ^^^^^^^^^^^^^^^^^^^^
  File "markdown/treeprocessors.py", line 310, in __applyPattern
    if not isinstance(node.text, util.AtomicString):
                      ^^^^^^^^^
AttributeError: 'AtomicString' object has no attribute 'text'

[waylan]

Well, it looks like I was wrong about this. Turns out any string should not follow that code path.

Thanks for the which make sit clear.