Parsing: Build FOR loops always with valid flavor.

This avoid issues, at least with string representation used in the log
file, if the flavor is `None`. Alternatively could handle `None` in
the model object, but them we kind of should handle also other invalid
flavors. There's also already typing using `Literal` that specifies
what are the valid values.

Fixes #5120.

atest/robot/running/for/for.robot

@@ -1,5 +1,6 @@
 *** Settings ***
-Suite Setup       Run Tests    ${EMPTY}    running/for/for.robot
+Suite Setup       Run Tests    --log log-tests-also-string-reprs.html    running/for/for.robot
+Suite Teardown    File Should Exist    ${OUTDIR}/log-tests-also-string-reprs.html
 Resource          for.resource
 
 *** Test Cases ***

src/robot/running/builder/transformers.py

@@ -370,9 +370,9 @@ def __init__(self, parent: 'TestCase|UserKeyword|For|If|Try|While'):
 
     def build(self, node):
         error = format_error(self._get_errors(node))
-        self.model.config(assign=node.assign, flavor=node.flavor, values=node.values,
-                          start=node.start, mode=node.mode, fill=node.fill,
-                          lineno=node.lineno, error=error)
+        self.model.config(assign=node.assign, flavor=node.flavor or 'IN',
+                          values=node.values, start=node.start, mode=node.mode,
+                          fill=node.fill, lineno=node.lineno, error=error)
         for step in node.body:
             self.visit(step)
         return self.model