Fix subtitle does not properly clear/remove from the chart

[cminn10]

Description

This PR fixes an issue where layout.title.subtitle does not properly clear/remove from the chart when subtitle object is not in place, or subtitle.text set to null, empty string, or whitespace-only values via Plotly.relayout().

Problem

When users attempted to clear a subtitle using any of these methods:

Plotly.relayout(gd, {'title.subtitle.text': null});
Plotly.relayout(gd, {'title.subtitle.text': ''});
Plotly.relayout(gd, {'title.subtitle.text': '   '});
Plotly.relayout(gd, {'title.subtitle': undefined});

The subtitle would remain visible on the chart. The only workaround was to set it to transparent:

Plotly.relayout(gd, {
    'title.subtitle.text': 'no text',
    'title.subtitle.font.color': 'transparent'
});

Root Cause

In src/components/titles/index.js - Line 163: The condition if(subtitleEnabled && subtitleElShouldExist) prevented D3's .exit().remove() from running when the subtitle needed to be cleared.

Additional changes

• Line 84: Handle null/undefined subtitle.text
• Lines 244 & 408: Add checks for empty D3 selections to prevent operations on selections with 0 elements

Notes

This follows the same D3 enter/exit pattern already used for the main title element (lines 147-157 in the same file).

[gvwilson]

Thanks for the fix @cminn10 - we'll see if we can find a reviewer for the next release.