[Processing][Needs-docs] Allow optional "value" in "select by attribute" algorithm

because it's not needed to run null checks (fixes #19469 which is actually reported in the wrong way)

(cherry picked from commit 4e1d29e)

Author: DelazJ

python/plugins/processing/algs/qgis/SelectByAttribute.py

@@ -106,7 +106,8 @@ def initAlgorithm(self, config=None):
         self.addParameter(QgsProcessingParameterEnum(self.OPERATOR,
                                                      self.tr('Operator'), self.operators))
         self.addParameter(QgsProcessingParameterString(self.VALUE,
-                                                       self.tr('Value')))
+                                                       self.tr('Value'),
+                                                       optional=True))
         self.addParameter(QgsProcessingParameterEnum(self.METHOD,
                                                      self.tr('Modify current selection by'),
                                                      self.methods,