There is a benefit to the pace of work becoming so hectic that I cannot post on CJEU case-law swiftly: others have analysis to which I can refer. In the case of CJEU C-804/19 BU v Markt24 GmbH, Anna Wysocka-Bar has posted analysis this morning (Opinion Saugmandsgaard Øe here).
BU whose place of residence is at Salzburg (Austria) signed an employment contract for carrying out cleaning work in Munich (Germany) for Markt24 GmbH, whose registered office is also located in Munich. The contract was signed in a bakery in Salzburg, where Markt24 also had an office. BU was never allocated any work, the employment contract was terminated and BU claims outstanding wage at the Landesgericht Salzburg.
The CJEU refers to Holterman to define employment  and holds  that the presence of a contract of employment is relevant for triggering the protective regime: not its actual exercise, at least if the lack of performance of the contract is attributable to the employer .
This issue was not sub judice however reasoning mutatis mutandis I would suggest the attributability or not to the employer be subject to the putative lex loci laboris per A8 Rome I.
Having established that A21 BIa applies, the question is how a ‘‘place where or from where the employee habitually carries out his work’ may be determined if no work has been carried out. At 41:
in the case where the contract of employment has not been performed, the intention expressed by the parties to the contract as to the place of that performance is, in principle, the only element which makes it possible to establish a habitual place of work (…) That interpretation best allows a high degree of predictability of rules of jurisdiction to be ensured, since the place of work envisaged by the parties in the contract of employment is, in principle, easy to identify
In casu, that place is Munich albeit  Salzburg might also still be an option given as A20 BIa makes A7(5)’s branch jurisdiction applicable (“as regards a dispute arising out of the operations of a branch, agency or other establishment, in the courts for the place where the branch, agency or other establishment is situated”). Whether the conditions for that Article apply, is for the court at Salzburg to determine.
The CJEU’s emphasis on predictability in my view also means that if a place is agreed yet the employee, without agreement from the employer, de facto carries out the work elsewhere, the agreed place must take precedent.
The CJEU also holds  that the employment title of BIA exhaustively harmonises jurisdiction: more favourable national CPR rules (in casu granting jurisdiction to the employee’s residence and /or place of payment of the remuneration) become inoperable.
An important judgment.
EU Private International Law, 3rd ed. 2021, para 2.278 ff.