templater: make pad() evaluate boolean argument (BC)
Otherwise it would crash if template expression was passed.
This patch unifies the way how boolean expression is evaluated, which involves
BC. Before "if(true)" and "pad(..., 'false')" were False, which are now True
since they are boolean literal and non-empty string respectively.
"func is runsymbol" is the same hack as evalstringliteral(), which is needed
for label() to take color literals.