Fix similar error on other keywords too

Author: aaltat

atest/acceptance/keywords/elements.robot

@@ -83,7 +83,7 @@ Get Element Attribute Value Should Be Should Be Succesfull error and error messa
     ...    Element with locator 'id=non_existing' not found.
     ...    Element Attribute Value Should Be  id=non_existing  href  http://non_existing.com
     Run Keyword And Expect Error
-    ...    Element 'link=Target opens in new window' attribute should have value 'http://localhost:7000/html/indéx.html' but its value was 'http://localhost:7000/html/index.html'.
+    ...    http://localhost:7000/html/index.html'.' but got 'Element 'link=Target opens in new window' attribute should have value 'http://localhost:7000/html/indéx.html (str)' but its value was 'http://localhost:7000/html/index.html' (str).
     ...    Element Attribute Value Should Be  link=Target opens in new window  href  http://localhost:7000/html/indéx.html
 
 Get Horizontal Position

src/SeleniumLibrary/keywords/element.py

@@ -55,7 +55,7 @@ def get_webelements(self, locator: Union[WebElement, str]) -> List[WebElement]:
     def element_should_contain(
         self,
         locator: Union[WebElement, str],
-        expected: str,
+        expected: Union[None, str],
         message: Optional[str] = None,
         ignore_case: bool = False,
     ):
@@ -93,7 +93,7 @@ def element_should_contain(
     def element_should_not_contain(
         self,
         locator: Union[WebElement, str],
-        expected: str,
+        expected: Union[None, str],
         message: Optional[str] = None,
         ignore_case: bool = False,
     ):
@@ -326,7 +326,7 @@ def element_should_not_be_visible(
     def element_text_should_be(
         self,
         locator: Union[WebElement, str],
-        expected: str,
+        expected: Union[None, str],
         message: Optional[str] = None,
         ignore_case: bool = False,
     ):
@@ -362,7 +362,7 @@ def element_text_should_be(
     def element_text_should_not_be(
         self,
         locator: Union[WebElement, str],
-        not_expected: str,
+        not_expected: Union[None, str],
         message: Optional[str] = None,
         ignore_case: bool = False,
     ):