FIX ConfigManifest regenerating every request if variantKeySpec is an empty array()

@chillu This is what I talked about last night. It turns out that `regenerate` was being called on `SS_ConfigManifest` whenever `variantKeySpec` was equal to `array()`.

Essentially unless you had a module installed that caused `variantKeySpec` to be non-empty the Config would regenerate on each request.

Before and after xhprof against a vanilla SilverStripe install that has a fully primed cache:

<table>
<tr><th></th><th>Run #51dca138cc06c</th><th>Run #51dca11fd851a</th><th>Diff</th><th>Diff%</th></tr><tr><td>Number of Function Calls</td><td>142,247</td>
<td>18,238</td>
<td>-124,009</td>
<td>-87.2%</td>
</tr><tr><td>Incl. Wall Time (microsec)</td><td>433,458</td>
<td>133,542</td>
<td>-299,916</td>
<td>-69.2%</td>
</tr><tr></tr><tr><td>Incl. CPU (microsecs)</td><td>426,059</td>
<td>126,969</td>
<td>-299,090</td>
<td>-70.2%</td>
</tr><tr></tr><tr><td>Incl. MemUse (bytes)</td><td>21,550,856</td>
<td>21,278,064</td>
<td>-272,792</td>
<td>-1.3%</td>
</tr><tr></tr><tr><td>Incl.  PeakMemUse (bytes)</td><td>21,669,480</td>
<td>21,395,448</td>
<td>-274,032</td>
<td>-1.3%</td>
</tr></table>