Fix invalid conversion in Element Attribute Value Should Be keyword

Author: aaltat

atest/acceptance/keywords/elements.robot

@@ -62,6 +62,8 @@ Get Element Attribute
 
 Get Element Attribute Value Should Be Should Be Succesfull
     Element Attribute Value Should Be  link=Absolute external link  href  http://www.google.com/
+    Element Attribute Value Should Be  link=Absolute external link  nothere  None
+
 
 Get Element Attribute And Element Attribute Value Should Be Should have same results
     ${attribute_value}=  Get Element Attribute  css=#second_div  class
@@ -70,12 +72,12 @@ Get Element Attribute And Element Attribute Value Should Be Should have same res
 Get Element Attribute Value Should Be Should Be Succesfull with non-ascii characters
     Element Attribute Value Should Be  link=Link with Unicode äöüÄÖÜß  href  http://localhost:7000/html/index.html
 
-Get Element Attribute Value Should Be Should Be Succesfull error and errors messages
+Get Element Attribute Value Should Be Should Be Succesfull error and error messages
     Run Keyword And Expect Error
     ...    Test Fail Custom Message
     ...    Element Attribute Value Should Be  id=image_id  href  http://non_existing.com  message=Test Fail Custom Message
     Run Keyword And Expect Error
-    ...    Element 'id=image_id' attribute should have value 'http://non_existing.com' but its value was 'None'.
+    ...    Element 'id=image_id' attribute should have value 'http://non_existing.com (str)' but its value was 'None' (nonetype).
     ...    Element Attribute Value Should Be  id=image_id  href  http://non_existing.com
     Run Keyword And Expect Error
     ...    Element with locator 'id=non_existing' not found.

src/SeleniumLibrary/keywords/element.py

@@ -25,6 +25,7 @@
 
 from SeleniumLibrary.base import LibraryComponent, keyword
 from SeleniumLibrary.errors import ElementNotFound
+from SeleniumLibrary.utils.types import type_converter
 
 
 class ElementKeywords(LibraryComponent):
@@ -414,7 +415,7 @@ def element_attribute_value_should_be(
         self,
         locator: Union[WebElement, str],
         attribute: str,
-        expected: str,
+        expected: Union[None, str],
         message: Optional[str] = None,
     ):
         """Verifies element identified by ``locator`` contains expected attribute value.
@@ -431,8 +432,9 @@ def element_attribute_value_should_be(
         if current_expected != expected:
             if message is None:
                 message = (
-                    f"Element '{locator}' attribute should have value '{expected}' but "
-                    f"its value was '{current_expected}'."
+                    f"Element '{locator}' attribute should have value '{expected} "
+                    f"({type_converter(expected)})' but its value was '{current_expected}' "
+                    f"({type_converter(current_expected)})."
                 )
             raise AssertionError(message)
         self.info(