[Quantity] Minor improvements & Make `quantityState` `null` when unit is missing

[florian-h05]

• Improve availability of symbol property (once again) by extracting it from the string representation of the Quantity instead of getting it from the Java layer.
• Log a warning when unit conversion fails (especially important for Blockly users).
• Return null from quantityState if Quantity would be unit-less (without unit) since there is no advantage over using a plain number. This is especially important for the UoM blocks in Blockly.

[stefan-hoehn]

Supports https://github.com/openhab/openhab-webui/pull/1864

Tested:

• Log a warning when unit conversion fails works as expected
• Return null from quantityState if Quantity would be unit-less ...

Both work as expected