Handle plot with `undifined`-title.

[dcaoyuan]

Test case:

//@version=6
indicator("Supertrend Strategy", overlay=true)

// Supertrend settings
atrPeriod = 10
factor = 3.0
[supertrend, direction] = ta.supertrend(factor, atrPeriod)

// EMA 200 for trend filter
ema200 = ta.ema(close, 200)
plot(ema200, color=color.blue, title="EMA 200", linewidth=2)

// Plot Supertrend
bodyMiddle = plot((open + close) / 2, display=display.none)
upTrend = plot(direction < 0 ? supertrend : na, "Up Trend", color = color.green, style=plot.style_linebr)
downTrend = plot(direction < 0? na : supertrend, "Down Trend", color = color.red, style=plot.style_linebr)

// Fill background
fill(bodyMiddle, upTrend, color.new(color.green, 90), fillgaps=false)
fill(bodyMiddle, downTrend, color.new(color.red, 90), fillgaps=false)

// Buy/Sell Signals
plotshape(direction < 0 and direction[1] > 0, title="Buy", style=shape.labelup, location=location.belowbar, color=color.green, text="BUY", textcolor=color.white, size=size.small)
plotshape(direction > 0 and direction[1] < 0, title="Sell", style=shape.labeldown, location=location.abovebar, color=color.red, text="SELL", textcolor=color.white, size=size.small)

in case of non-title bodyMiddle:

bodyMiddle = plot((open + close) / 2, display=display.none)

It will fail on test:

PineTS/src/namespaces/utils.ts

Lines 5 to 7 in a073796

	function isPlot(arg: any) {
	return typeof arg === 'object' && arg.title !== undefined && arg.data !== undefined && arg.options !== undefined;
	}

And causes series consequence. For example, cannot be correctly parsed by: parseArgsForPineParams during:

PineTS/src/namespaces/Plots.ts

Lines 426 to 434 in a073796

	export class FillHelper {
	constructor(private context: any) { }
	param(source: any, index: number = 0, name?: string) {
	return Series.from(source).get(index);
	}
	any(...args) {
	const callsiteId = extractCallsiteId(args);
	const _parsed = parseArgsForPineParams<FillOptions>(args, FILL_SIGNATURE, FILL_ARGS_TYPES);
	const { plot1, plot2, color, title, editable, show_last, fillgaps, display } = _parsed;